To see what you can sell any gun-related thing for, check with Gunbroker.com. Here is a link to their reloading supplies part of the site.
As you can read, somebody has 6 bids on 500 .45 ACP once-fired cases, and the high bid is $21. Watch what things actually sell for, and you will get an idea of how much you can ask. Dirty, mixed headstamp brass with unkown origin off a comercial range floor is not going to get the best price. Brass that you fired yourself from factory new ammo will get more money. If you deprime and clean the brass (especially if you tumble it wet with stainless pins) will get you a little more. But, DON'T SIZE the brass, as that will make a lot of guys pass it up, because they don't know how you sized it and would prefer to do that their own way.