I've bought a few "in-game" items before. The most I paid was $25,- in US-dollars, luckily that was when the dollar was so low that it didn't translate too much more than a drink and a sandwich here; though I do have to say that overall it's never really worth it in the long run. It's somewhat like spending MS-points on Avatar items on Xbox Live; it's all fun and well, but eventually you'll have this though occurring to yourself asking you "Why?" (And I'm guilty of this too).
Other than that I've bought some Folklore DLC; I know not specifically "Items", but frankly the major reason I purchased them is the additional costumes for Ellen and Keat. The additional missions are all fun and well, but they add very little to the game.
So... it really depends; if I absolutely adore a game (Folklore, Mirror's Edge and a few more) I wouldn't mind spending anywhere from $1,- to $40,-. As long as the price is somewhat balanced with the worth. With Folklore I paid about $15,- in total for 4 pieces of DLC which added quests and costumes. That's good value; I would've even paid a bit more.
Short answer, yes I have... but it's not always worth it.