Really great question! It comes down to a comple things. Who your target demographic is, what your overall goals are (sales, leads, traffic generation, likes, follows, branding and so on).

You'll find that most agencies will sign you up percent of advertising spend contract. This is common it normally ranges from 20-40% depending on budget. 

A good advertiser or agency will sitdown, nut out a story with the client, their goals, who their demographic is, provide plenty of ad copy variations and visual varations to A/B test then work on a results based payment. 

There isn't one right, or wrong answer for how much it costs though. It does say you're after a price range. I'd say it's $300-700 setup fee and a 20-40% ongoing fee of your budget for a standard agency. For a good agency that'll generate better conversions and focus more on your goals will range from 600-1200 setup and then work on a percentage per sale (higher ticket items are harder to sell, where lower ticket items are easier so this works out if you're selling a car vs a phone case). 

I hope this helps. 

Following on from the other answer by Brodey, there are so many variables to consider and no one answer to this question.

The targets that you want to achieve would need to be assessed and objectives very clearly defined. There are a lot of variations of Facebook Campaigns with ad types and approaches for each depending on those goals that are set initially. In addition, you need to understand EXACTLY who you want to market to using the N.O.C Method (Niche, Offer, Copy). That way once you do launch any type of campaign, you have this very dialled in so you are able to get started with a test budget to see what works best. 

The prices for the set up, management and optimisation of these campaigns will vary depending on what is required  to execute the campaign effectively eg: (building a high converting funnel, CRM and payment gateway integrations). You may be looking at $1,000 for a simple campaign or up to $5,000 for full management.

Hope this helps!

I understand that the campaign is for a small business, hence, a small advertising budget. I recommend partnering with a service provider that caters mostly to small businesses as they understand the dynamics of a low-budget ad campaign and know how to maximize results. While there is no definite answer to the question, it is best to get an estimate on a few service providers. However, it is best that the objectives of your campaign, audience, content, and other details area already clear in your mind to get an approximate quote. We provide this kind of service at ProcessCorp. Happy to assist you in any way.