Depends of how much power you want.

Single turbo systems can give 200-260 BHP
Twin turbo syems can give upto 330 BHP
Supercharger systems can upto 250 BHP
Enlarged engine capacity can give upto 225 BHP

I'm sure you can get more power but you are starting to talk very serious money. The cheapest is a second hand turbo setup as made by turbo technics but ensure you have all the parts.