On my 330xi I have a remus muffler and a resonator delete. I like it and isn't infamously loud like a muffler delete, just right for me. I bought my Remus for $200 used. Other then exhaust, you can do a K&N air filter drop in. I'm doing that this weekend. http://www.e90post.com/forums/showthread.php?t=21590
You can get air scoops also
There isn't much for performance under $100. That's all I have in mind, I'm a college student and budget is always on my mind, that's all I've got for you. good luck!