Best bang for the buck.
1. K&N panel filters (~$100)
2. Cobb (or similar) smooth intake tubes (~$200)
3. ebay test pipes (warning -- raspy!) (~$75-$150)
4. Nismo (pass on the S-tune, get the OEM "take off", picked up used -- ~$300-$550)
Total cost: Under a grand
Results:
Note 1: That is actually a bit lower -- add another 3-5 whp/wtq for the tubes. That dyno is before the tubes were added.
Note 2: If you are worried about rasp, pick up a set of used resonated test pipes for a bit more $$.
Note 3: Most of those gains were from the K&N and TP's mods, so if your budget is really tight, just go with those.