The general consensus has been for a while that a complete buyout of rights is 3x the cost on commission. I want to say it came from the illustrator's handbook, but I can't recall at the moment. Given that I really don't like selling my rights I bumped it to 5x. Mostly to scare people away unless they absolutely want it. I mostly do sketch commissions with a max price of $110, so if they want me to never post it the sketch has to be worth $550 to them.