(CornyGuy)
Pricing of snakes needs to be based on several factors. Morph, age(you've stated an adult), and where you buy from. Breeder, hobbyist, etc. As Skunky said, check out this site- www.cornsnakes.net They have a lot of different morphs with their prices.
The only problem with looking at prices on a website is usually that the prices listed are for hatchlings.
Adult prices can vary greatly depending obviously on the morph, but also on the gender and "quality" of that morph. For example, baby Lavs can run from about $75-$80 (give or take), but I've seen an adult female being sold for $400. But, I also scored a pair of almost adults (they'll be ready to go next season, though they probaby could breed now but I don't want to breed too young) for $210. But, that was at an expo, and it was extremely cheap.
You can always find deals, and you can always find what may seem to be overpriced individuals.
I would find out what morph it is and we can try to give you a target price.