What they cost you divided in half is the most you can usually get.
Depending on how old / obsolete they are will drop the price more.
Some rare / collectable things go up in price.
The only 'real' way to find what your parts are worth is to search eBay and craigslist and see what others are selling their same parts for. That is your competition and you have to lower your asking price or your stuff will never sell.