If you want any sort of noticeable gain and want to stay cheap, sell your car and buy something faster. These cars will only go very fast with forced induction of serious work.

 

Most "bang for 'cheap' buck" will come from a full exhaust, headers and cat. back. Of course if you adjust your exhaust, it only makes sense to adjust the intake, so a CAI is probably the way to go there or at the very least a K&N.

The next thing would be to maybe port and polish the heads, but then it starts getting expensive from that point on.

Unless you do just go crazy, there is only so much you're going to get out of that car with it naturally asperated.