here's some of the cheapest ways to increase and restore horsepower:

cold air intake - these work wonders. they increase the amount of air that flows into the throtle body and increase MPG's too. the CAI filters are washable and you'll never waste money on disposable filters anymore. after 2 years, my AEM has paid for its self.

spark plugs - make sure you have clean, working spark plugs. it really makes a differance.

some people may say headers and exaust, but with an exhaust, you'll be lucky to get an extra 3 hp to the wheels.