In the end, the value of
trading what you don't need for something you want is only settled in your own mind.
If you want to convert what you don't need into cash, check out some of the postings on
http://www.reloadersauction.com/If you are more interested in trading brass you don't need for something you can use, pick a number to start and negotiate a settlement.
Personally, I'd rather give away brass I don't reload to friends, and hope they will remember me when they have something I need.
I understand that some brass (split, crushed, rimfire) is best sent off to a recycling center, but know that it will be turned into bells, door knockers, and faucets. Recycled brass almost never is made into new brass cases. Spent brass ammo cases are only kept in the arena of shooting sports if we recycle it within the sport of reloading. Even rimfire cases can be swaged into jacketed bullets.