I am a fashion shooter like the dude you are talking about. Buyouts are expensive. He is giving up his copyright to the images, and you are going to be making a ton of money off those images. What is that worth.
In the fashion industry, you triple your rate for a buyout normally. I have no idea how porn shoots pay.
It just seems like he is doing all the work and you are getting all the gain. It's his equipment, time and retouching time. $300 -$800 is a slap in the face imho if he is any good at all.
my 2
