The Poorman HAI
1. Go out to Advanced AutoParts (or any other local auto parts store) and pick up a generic cone filter. The one used here is the APC Intimidator.
2. Remove the top portion of your airbox. I recommend you not remove the bottom as it can keep your intake from falling off (as many people have had problems with).
3. Carefully pry the brass plate located inside the top portion of your airbox out. It is held in by 4 screws in each corner (if you look carefully you can see the screwholes in this pic). This will serve as the adapter plate which you do not need to buy.
4. Attach the intake to the adapter plate and tighten it. Then attach the adapter plate to the rest of the intake tubing. Make sure the adapter plate is flush and you have not bent it in prying it off otherwise air will leak and you will have to buy a new one.
5. Place Air Temp sensor (previously plugged into airbox) anywhere in the intake system where it can get air. Then attach everything back together and tighten all the metal loops (braces).
6. Relax! You're done and all you bought was the cone filter.
I think this is the cheapest way imaginable.