Mileage and HP usually don't go hand in hand. It's kind of a trade-off.
For that budget I'd strongly recommend a mail order tune. Out of all the things I've done to my 4.3 truck, the Black Bear Performance tune made the biggest difference by far. Much more power and noticeably more fun to drive. It won't help your mileage, but it won't hurt it much either. I can still average over 20MPG when I keep my foot out of it.
EDIT - Whoops, sorry. Just realized you had a TBI truck. Disregard, mail order tunes are much easier for the Vortecs. You posted this in the 96+ section so I assumed that's what you were working with but then I saw the username.
