I have bought a couple, got burned once the very first time. I make the person go through a series of random verifications such as putting a silly word of my choice on 5 random posts if my choice, screenshots within 1 minute with correct times etc.. there's many things a person can do to verify.
I only use Escrow.com Escrow Hill or Trade Guardian. Usually by the time you get to the 5th or 6th question or ask the person to verify their PayPal address the scammer backs out in my experience.
As for OP I'd say if you have 100,000 followers and you're getting 1-5k likes per post, more like $400-$600 is you're getting 10-20k more like $1850, and if their at least 95% organic, a tad more.