Depending on the model make, check out GunBroker.com for an average value. I would think anywhere between $500-800 depending on its condition and any customizations. Or take it to a gunshop for another opinion.
"There is no hunting like the hunting of man, and those who have hunted armed men long enough and liked it, never care for anything else thereafter." - Hemingway
The greatest ignorance is to reject something you know nothing about.