Cheapest Horsepower Gain??

What is the cheapest horsepower gain I can do??

Since you didn't specify the model and engine size, try a cold air intake first. Relatively inexpensive and will boost power a couple horsepower. If you have a GTP you can get a smaller pulley for the supercharger along with the cai and see some impressive gains, also opening up the exhaust by removing the u-bend will help alot on the GT or GTP.