I am not really sure about the costs. I went back with stock pistons and didn't even port out the exhaust ports. The biggest problem is the intake system. It is very restrictive. Cams are expensive too, but you might be able to have someone regrind the stock ones. The can take some off the heel and that will raise the lift. You can buy better cams, but it is very expensive.
The engine has about 200 HP stock. I know there is a site where the engine can be stroked and brought up to either 3.2 or 3.4, not sure right off. If you could live with premium, then higher compression pistons would help a bunch.
The whole thing as you know is breathing and it is hard to beat the turbo in this area. Nitrous is going to be a short burst at best and there is a pretty good range of shots to use, 50 HP and up. I used to race a Celica and shaved 150 thou off the heads with no problems. The Supra head looks like it could take at least 100, but I couldn't be positive.
www.horsepowerfreaks.com is a good source of performance for the N/A and the turbo. Again they are pricey. I would go with the headers, good free flow exhaust, at least 2 and 1/2 " without cat if you can, Mill the head a bunch, retard the stock cams if you can't afford new ones, that will give you more upper end HP without sacrificing lowerend too much. Advance the timing as much as you can with the best gas you can. Block off the EGR valve and try an electric radiator fan.
All these things will add HP and not cost too much. You will feel the gains and the car will be more fun, but eventually you will want a turbo, but with a rebuilt engine you can get more money for yours and look for that turbo in the mean time.
Sorry I am not more specific as far as exact gains and where to buy parts. The Supra is an expensive machine to upgrade, but I guess HP does cost money, but way more than a 350 Chevy!
Russ