if you want big hp is gonna cost big $$.
figure around $10k-15k
for that price, I would just buy a car that starts off with a lot more hp first.
yup for 15k plus trade in you could get a GTO with 350hp. for a few hundred more get new heads and cam shaft and jack it up to 400-450hp. or for that 15k buy a chevy ramjet crate motor and figure out how to put it into your car and make it RWD. or if you need a winter car with better traction in the snow and ice for the same price you can get an AWD wrx sti 300hp. or for only a few grand and a trade in you can get a dodge neon srt-4 and with a turbo upgrade you can make like 350. these are all pretty ugly cars and I've never driven any so dunno about the handling but if you want cheap HP go for these.
think the celica really shines in looks and handling so concentrating on those aspects are good suspention visual upgrades and maybe alittle more HP if natural go Intake
and exhaust cams when they come out with 'em. or a mild turbo so you don't have to upgrade everything like stage 1 c2gas on a GTS would give probably the cheapest and most reliable gains for about 4 grand and no need to upgrade tranny or internals.