Exhaust wise, the manifolds are the biggest restriction(as far as i know). So, ported manifolds go the furthest as far as hp per $ when spending money on exhaust. I know there are a few people on clubgp.com that do vendor quality jobs for $150 or less.
A fender wall intake or cold air intake is also a nice hp per $ mod since it can be made pretty cheap.
But if you want some nice power, try some higher ratio rockers.
The cheapest price new you'll find is $280 + $25 for new torque to yeild rocker bolts and gaskets:
And it'* not really needed if you have lower mileage but, new springs will help kill valve float.
There only about 70 bucks or so at gmpartsdirect.com.
If you want the most power, cam it!
but that'* big $