Not sure about "cheap" but I'd suggest adding a chip to the engine. If you go to ebay and type in "1998 ford mustang gt chip" - you should see many merchants selling chips for that car. Other than that, I'd also suggest new rims and tires - always helps with the handling.

good luck

lstein

It not very cheap but a supercharger will be worth the money in horsepower increase.  A cheaper solution is NOS but engine failure is common with these system.  Search e-bay motors.

A good mandrel bent true dual exhaust with an x-pipe  and a K&N FIPK Kit will give you the most bang for the buck. I like Flowmasters , but Magnaflows flow more freely supposedly.