If you have only ever bought CPA traffic I would say you have zero experience and prepare yourself for some frustration right now. Because there will be a learning curve.
You certainly can't pay based on CPA which means you are going to have to figure out how to convert it all on your own. Test and find what demos, keywords, ads and landing page all go together to give you a cheap CPA cost. If you can't do that on your own you will need to stick to paying a higher CPA cost and let the Facebook gurus promote the offer for you.
As far as what spend is needed to optimize an offer that all depends on that payout of a conversion and how much you already know about the best way to promote it.