I like Ike('s) suggestion that the best HP per $ power increase for a 2.7L comes from an intake and exhaust mod. I drove Woody's build that had a 2.7L engine with a 996 intake, exhaust, and tune. I don't have numbers to quantify it, but it felt like a big improvement from a stock 2.7L and is a lot cheaper than putting a full 996 engine in.
For an Audi swap, I don't think you can do much better than what truegearhead did with a twin turbo 4.2L.
http://986forum.com/forums/performance-technical-chat/77421-time-trial-build-thread-aero-widebody-any-other-nonsense-10.html
WizardSmokey took a
beater Boxster, cut through some unneeded metal on the back end and put a Maserati engine in.
If you want significantly more power that's affordable by just adding some simple bolt-on parts, you have the wrong car. If you are willing to do your own research and create your own adaptations, there are a lot of possibilities.