[vrmilionzx]

I'm with NoD, mostly.
My "fun car" is my s14, it's fully built, mid size turbo, and puts down some solid numbers. It's pretty efficient on the highway, gets me a little over 30mpg, which is solid when I'm driving normally (65-75mph, occasional boost to pass), and around town is still just about what the epa estimated, though that depends on how heavy my foot gets.
More than anything, you have to take into account what kind of personality you have - because it's a hell of a lot of fun to stomp the pedal down when it results in feeling like you're taking off in an airplane. It's terribly addictive.

Doing a build "right" means you're making it more efficient, ideally across the board, so there's room for gain (mpg) there. Thing is, you'll spend a lot more money than you'll save in any reasonable amount of time, like NoD said. Doing it right means, bare minimum (assuming you want modest power gains with reliability), a manifold, turbo, way to mount exhaust to turbo, small intercooler, tune (this can get costly), potentially bigger injectors, wideband to monitor, oil delivery to turbo, method of monitoring and controlling boost, potentially a new fuel pump...... and then, it REALLY is addictive and fun.
It can be done, but as the saying goes: Fast, Reliable, Cheap - pick two.
I also think your money will be better spent on aero mods, or making it run more efficiently. If you're looking to do fun mods, lightweight flywheel, take out those back seats if you don't use em, lighter wheels, stuff like that.
I'm not a fan of PT's, but that's just my personal taste, so I don't know too much about them.

But this is working under the assumption you want better mpg, not loads of power. If you want anything more than modest power gains, consider building the motor, and going all out. Do it right, or do it twice.