Price PLunge has Def Jam Rapstar for $2.00 plus shipping, which for the cheapest comes out to under $3.50. $6 for the bundle with mics, but I have a ton, so I didn't need anymore. By all accounts it's a really good game, and I've been wanting to grab it for some time and this is the perfect chance. The game is not supported with DLC anymore, but I'm ok with that at this point. Also of note the songs are censored if you worry about that kind of thing.
Edit: My order already shipped! Yay.