i have a 91 ls congratz is it 5sp.
lets get these out of the way first how healthy is your motor, how many miles...
u wont find a exhaust for 100 i wish i could, for a brand name your lookin in the price range of $400-$600 (hks for a 91 is 6 something outragous) anyways i have seen dynos where a brand exhaust can get 20 hp (hytech very expensive) same with headers they are around $300-$400 i have seen dc sports is a good company.
if your motor is old save out and rebuild or if it about to die get a motor swap in future thats the best way to get a decent difference, us 91 ls guys dont have the magic called vtec, if i could i would send u my back copies of importtunner they always dyno testing new products, oh forgot to mention there are ways to cheat the dyno so u cant go buy what the company says. hope this helps and chech import tunners website(dont remember the site name) they sould have some dyno tests on there good luck and have fun with the 91