Is there a way to differentiate between paid and free apps in iTunes purchase history? I want to know what apps I've paid money for, as opposed to free apps that were downloaded. 

I need to know this because I gave my little brother my old 3GS to use as an iPod which remains attached to my iTunes account because he doesn't have a credit card or an income in order set up an account. I notice that he downloads a lot of apps, games in particular, and I'm worried because I've tried a few that seem like they could easily charge for. I often download apps that I pay for, so often that I wouldn't know if he was buying things without my knowledge. 

Every now and then I notice a charge from iTunes that I don't remember making, so I need to know what I've paid for. Is there a way to show what apps I've actually spent money on?
In addition to keeping your email receipts, you can also view your entire purchase history by viewing your account in iTunes. If you're signed in, click your account name in the top right, and choose the "View Account" option. On the following screen, the third section down shows "Purchase History", with a link on the right to see all your orders. You can see each order, listed chronologically, and jump to specific months and get more detail about a specific order.