1. Air Filter proberly the cheapest.
2. Exhaust.
I've got two but haven't fitted yet, Olly98 has built me a T-Joint, but I may have to change because of the Filter from the cam area bit. I've been told that you really need the filter to be inside the air flow to suck the c**p out of the engine.
3. Remove parts of the car that's not required, that's really cheap, lol.