They can be expensive, depending on their rarity, how long they ran for, and where they came from. Like for LoZ: OoT 3D, since it had such a limited print run at first, it skyrocketed to some crazy prices until Nintendo re-released it for 20 bucks, or before that, if you didn't want to spend your college fund on a physical copy, it's on the eShop for about the price it went for-40 bucks.
If you're looking for games like Fire Emblem though, you might wanna stick with Amazon. It can get crazy on eBay. o__o;