Talked to 6 different individuals and small agencies, all of whom seemed like they knew what they were doing. I chose one that's based in SoCal and is a small team of 3. Total cost will be between 8 and 10 grand over three months.
There was consensus among the 6 that I'll need to spend at least $2000/mo, maybe more, for a few months to really dial in the receptive audiences and get the CPC down.
There was also consensus that my business model can probably work on Facebook; lots of models don't. But they also said that the only real way to confirm this hypothesis is to just do it. And to do it right, you have to give it a little time and put real money toward it.
So while 8 grand is a ton of money for me as a bootstrapper, this seems like the quickest route to traction and revenue.