So I've been browsing E-bay for used eheim canister filters. And I'm wondering - how much value do the Eheim filters hold? Is half-price a good estimate for what I should pay for a used eheim? Or less than that? More than that? Just curious. Seems there's prices all over the board, but many things seem overpriced to me... I figure if I'm going to spend that much I may as well spend the extra 20 or 30 and buy new, etc?