Well I got mine to roughly 125-130bhp margin my performance mods where:
Fast road cams, lightened balanced botom end inc flywheel, ported flowed and smoothed head, titanium valve springs, pipercross viper induction kit (custom ram cold air intake), Supersprint stainless straightrough exhaust head to back with de-cat (was still MOT legal! :slayer

. Put a remap / chip in their and your talking 140bhp but I didnt get chance to do that

.
All that above cost me roughly £1000ish the most expensive was the labour intensive tasks like the lightening and balancing. It was well worth it though it had a mighty induction roar similar to that on a BMW M3 I was comming through a tunnel and let it rip in second and got pulled over by cops at the overside and he said your car sounds amazing just like my M3 at home, what you packin under that bonnet

. Thinking about the engine noise still makes me all warm inside

.