Do the stealth. 30 dollar filter and a fun whoosh and some better acceleration. Although little it's worth it. I think it was the best thing I did when I started(that gave me the mod bug) That and I had my stock pipe cut shorter and welded a new larger ford tip on it. Ford accessories website. Go to the v6 fusion it's two tips for like 50 bucks split it with a fellow focus friend it looks good. Check out my gallery. Some of the older pics before I got my full exhaust and wheels.
The absolute biggest bang for your buck power adder available for the focus is nitrous. Its the only way you can more than double the hp of you car for under a grand if you spend your money right. but i still wouldnt call it cheap power. in the world of the focus unfortunately there is no such thing as cheap power unless the amount of power you gain is 1-2whp for $50-$100.