How much trade-value would I get for Black Ops in GAME, gamestation and HMV?

basically, I am sooo desperate to get some money together for Pokemon Soulsilver and I need to trade in Black Ops (xbox360), since I never play it. So, I would like to know how much money I would get if I traded it in in any shop (I live in Oxford, England - nowhere else) thanks a lot if anyone can help, I need the money!