I own a 99 C5 with 100,000 miles. For about 800 dollars I added some upgrades recently. I bought a haltech intake which gives an instant 10 HP. Very easy to install. Only takes about 20 minutes and a screwdriver.
The other thing i did was swap my exhaust from stock to a Borla. It looks a lot nicer than the stock. I took it to meineke to swap it out and it cost about $90 to do. Doesnt really do anything for HP. However, at around 1700 rpms and higher you'll definitely notice your car clearing her throat. At lower rpms it has a nice mucle car rumble. Hope you find what your looking for.
One last note. With the exhast and intake dyno'd you can get up to a 30 HP gain. Which i believer is the cheapest and easiest HP money can buy. www.southerncarparts.com