The size and sex makes a slight difference IMO. When you say half of what they paid, I partially agree. If you needed a home and the accessories, half the cost would be fair for them. The snake has grown, and had to fed and maintained. If it is this years hatching, I wouldn't pay more than the owner did and half the price doesn't seem unreasonable.

Lets say you have normal female of breeding age. She would be worth more, because of the fact she could be bred with a morph and produce morphs. So the value really is determined by the sex, the size, and the type. Realize that if it was purchased from a pet store, I definitely wouldn't pay much as it is likely WC.

I would ask the person how old it is, where they bought it and some more about the snake. Watch it feed if possible, and ask if the snake has had a fecal exam from a vet. Check it over externally for mites, loose skin, scars et... The snake should flick it's tongue a lot when being held. If a home is not supplied make sure to set up the proper environment first. Check out the living conditions of this snake. Is the cage clean, does it have proper heat and a t-stat et.

I have rescued snakes before where an adoption form and fee was required. Usually in those cases an adult's adoption fee was about the same as a hatchling would cost. If you really want to rescue a snake, there is an over abundance of BP in rescue shelters. The adoption fee would be minimal, and the fee will help the shelter continue to rescue animals.