The only half way cost effective way to increase horsepower is by replacing the cat and exhaust. You should be able to get it done for about $500 installed.
Don't waste your money on any sort of intake system or air filter. You'll never notice any difference with those.
After that, it gets expensive to get more power. The general rule of thumb is $100 / 1 hp. IE, 50 - 60 hp from a supercharger = $5,000 - $6,000, that's about right.