Nothing CHEAP is realiably going to give you a lot of power.
The Comptech is a supercharger kit, also Vortech makes a supercharger kit for the s2000.
Here's a link for a turbo kit:
http://www.ultimate-racing.com/Produ...TurboKit.shtml
You could always use NOS.
While it is cheap to install, it gets expensive to run all the time (cost of gas re-fills). Also because they can be quite easy to upgrade the amount of power, many people tend to keep increasing the amount of NOS until it exceeds the cars limit or do not set it up correctly (eg fuel delivery) and they end up seriously stuffing the engine up.
M@