Don't forget gazelle.com

I'd get the market value via Craig's list and eBay, but don't forget to try gazelle.com. They buy used stuff, and you can get a price there as well. I'm frequently impressed by how much they'll pay, most time more than I really feel that an item is worth.

There really isn't a blue book or a pricing authority, just the "going rate" *smile*. Whatever people will pay for it (the final auction prices on eBay, etc.). So if you're going to donate stuff, that's how I'd get a price to put on it.