Recently getting a few offers from local businesses for doing their marketing.
How much would you charge if you were doing facebook marketing for local businesses?
Thats a really broad question that needs more context.
What can you realistically deliver for the business? Your billing should reflect the value you deliver not necessarily time spent. Obviously that can go either way....
What specifically do they need you to do - deliver leads, sales, brand?
What do success metrics look like?
How will you measure success?
What's the strategy look like? How big is the TAM? Are you sure it's just local customers?
What can you realistically deliver immediately and across the next month, qtr, year etc?
What are their expectations - v important.
Is there a baseline?
What's required from a budget perspective to deliver on your promise?
Is facebook the right channel given the objectives & investment available?
What's the clients understanding of the space? Do they require extensive education? Do they even care?
Where are the resource gaps - what can you do, what can't you do, what do you need to learn?
What are the expenses (actuals) to your business that you need to cover off?
So many questions. I don't think anyone can give you a blanket statement on your pricing position without you briefing us out a bit more.
Also what do you want to get out of the relationship besides the monthly invoice? Are you looking to build out a consulting co/agency, do you want cred for your business, will you use them to bunny hop to the next guy & so on.
My point here isn't to slam you. You just need to think through this which will ultimately lead to the answers you seek. You may have given this extensive thought already, however to what extent is unknown given the question.
I thought I could just get a percentage of the total spending budget of the local client.
In my situation it is a dentist (local business) who needs more customers for his teeth whitening service.
You asked some great questions which I cant answer at the moment. I will be discussing this soon with the dentist.
Basically i will be doing some testing on adwords and facebook, landing pages will be created to test everything out. Sales and leads will be tracked ofcourse.
The Client doesnt have a clue on whats possible on the internet but likes to do this as a try-out.
I am not really trying to build this out as an agency, just want to run a few local clients next to my own affiliate marketing campaigns.
Its not a lot of info but i hope you can push me in the right direction...
I would also take a % of the overall budget.
An entrepreneur gave me the best answer to this broad question.
The real question you have to ask yourself is: "how much money do I have to charge to be motivated to work for someone else?"
What is the minimum rate that will make you move your ass? 
As a consultant, it's very very very hard to be motivated to work for others.
You'll have to deal with a lot of shit (paperwork, pressure, useless business meeting, invoices, contracts etc.), and every second you'll spend working for your customer, you have the unpleasant feeling that you're not building a long term asset for you and your loved ones.
Every second you spend to make others than you rich, is a time lost for ever 
My suggestion is: think about it!
At what price can you stay motivated to waste your time to build someone else's happiness?
PS: that's why when I'll be financially free, I'll charge ridiculous amount to discourage potential customers to work with me 
Also, keep in mind you have very important skills.
The industry is full of consultants that have no fucking clue when it comes to make a campaign profitable.
All of them have read books and blogs, but they're not so many to really be able to make a living from their skills...
IMHO, these skills are golden! They can't be transmitted for a discounted price.
Remember: those who know are playing the game. The others are consultant, teachers or bloggers 
PPS: I'm a consultant myself lol, but I am pretty realistic 
% of spend is IMO broken business model, b/c optimizing their camp actually means less $$ for you.. I mean unless you are super honest and have no greed hormone it's not gonna work.
I actually find it really funny that most agencies work like that. It just doesn't make sense for customer.
I would never do client business again, it's too much hassel for people that don't know what they want. Not worth the headache for me.
I usually get them commit to an ad budget + a consulting rate for a contracted period of time.
My consulting rate is $140/hour. I usually do contracts in 1,3 & 6 month intervals.
Always run them through the education of 'it might not make money immediately' and make them sign off on that fact that there is no guarantees in advertising. Estimate your hours to set-up and maintain the campaign, then add 25% for unpredictable issues and meetings etc.
Also make sure there is a clause in your contract that indicates that if their needs change dramatically, you will charge X dollars or renegotiate it as a separate project.
You can also track sales made & take a % of revenue.
It all comes down to managing client expectations. I charge ALOT to consult and charge a 20 hr minimum. Not for everyone but the people that do hire me know what to expect and I have not had one unhappy client. On the same hand there are several potential clients I have not taken because I felt they were unrealistic. Normally a quick 10 minute initial conversation will let me know if they are a good match for consulting or management.
From someone who got into IM doing local biz marketing (and made it work)...
If you have the skillset to do AM, or the drive/vision to create your own products and company (not a services company), then absolutely 110% do that man. It's faster, funner, better ROI, and you don't have to deal with anyone else's headaches (because they will infect you and drain your soul).
Best case scenario doing local biz IM: You build a real business doing local biz IM and then replace yourself. Time frame to do that? Depends on your startup capital and whether or not you've done it before (build and exit) - if you're inexperienced but good at motivating and training people, expect at least 2 years before it's even remotely stable.
Now, you may be in a situation where you see the money that can be made in local biz IM and that excites you ($4k-$11k+/month per client were my rates), especially if you're not making that much on your own already, but just know that that is a slippery, slippery slope my friend. Today you will get the client, tomorrow you will serve the client, the next day you will want to get more clients to get more money to automate what you're doing for the first client... and BOOM you are officially the CEO of a local business marketing firm. Is that what you want?
Then again, you might be the kind of person who just wants to get that one client and you would be happy working with just them. You have impeccable time management skills and can get the job done for them quickly, using pre-trained outsourcers to complete various tasks (heavy lifting) so you can just create and run a camp for them and deliver ROI. You then take the money you're making from that ONE client and reinvest it into your main business (AM or otherwise) and eventually you no longer need that client. You eventually pass them on to someone else who can get the job done, better and cheaper most likely as they really WANT to be in local biz IM, and you get on your merry way.
Final thought: If you're the kind of guy who gets into something and has an insatiable, unquenchable, relentless drive to be the best at it, STAY THE FUCK AWAY FROM LOCAL BIZ IM. It is a timesink that will shrink your mindset and force you to deal with people who are more or less relying on you to make them successful.
At the end of the day, do what you WANT to do. Don't chase the money man. "If you had all the money in the world what would you do with your days to better the planet?" Odds are the answer to that is not local biz IM.
My $0.02.
Kevin it sounds like you aren't making as much money as you could be, by figuring out if they're "worth it" with a 10 minute IM Chat. No offense intended at all, I think you've got more potential there.