Cheapest is to just by an ECU from
www.FWDperformance.com/store/ only stage 1 if you are not running an intercooler. If you add an intercooler(just one out of a junkyard, and make your own plumbing) you can get the stage 2 ECU running 14 PSI. Just getting the stage 1, and doing nothing else with get you probably 20-30 HP. Adding the intercooler and stage 2 ECU should get you 40-50 HP, and that is a lot. You might want to consider getting a better flowing exhaust system though. There will be less turbo lag and much better performance anyway. If you get 3'' turbo back exhaust from FWDperformance along with adding an intercooler and stage 2 ECU I guarantee you will yield atleast 50 HP which is 1/3 of the HP of the stock engine.
The exhaust is kinda expensive, 3'' is 400 dollars, and 2.5'' is 300 dollars. 2.5'' is good up to 250 HP or so, so it will be OK for you.