Sure, you got yours for cheap, but maybe he's not as big a fan of Rob and Fab as you are. He may not want 200 copies of The Remix Album.
Yeah, because everyone knows products with small supply = lower prices always
If you've actually used Amazon to purchase CDs before, you'd probably know that most any album can be purchased for under $5 no matter how big the album is. Look up the top 10 albums in the Rolling Stone list, you will find all of them for under $5 used (usually around $2 actually). If you want to "step up" to the remastered version, you'll pay maybe $6 to $8.
There's more copies of big albums floating around, so they're less expensive. LPs are a different story, but CD wise? You will find most good albums on CD for CHEAP used.
Anyway, stores where you can buy stuff in real life are almost always better. You'll have the album right away to listen to, no shipping costs, you can see the item's condition instead of relying on a picture and/or text description, etc.