What would be the best way to boost performance in my mr2?

The two modifications that are within your budget for this situation would be changing to a performance INTAKE and EXHAUST. Not only is this a good boost (maybe gain 10-20 hp) but you'll be saving weight as well!

More weight savings relatively cheap (or free)....

- remove the sound deadening material below the floor carpet (serious it's like lead)

- change to lighter rims (this is a big plus because unsprung weight such as brakes/rims have a larger impact on acceleration than your sprung weight)

- have a simple stereo

- if you're really serious you can always remove the spoiler and ground effects

I'd suggest throwing in a strut bar front and rear if already not standard.

Any upgrades beyond intake/exhaust will require more extensive work and money (ie: along with upgrade of fuel system, brakes, suspension, etc $$$$$)

The cheapest way to improve performance is probably to change your air intake. Be careful of which brand you buy though, so you dont get a crappy one. Air intakes will only cost around 200$ for a pretty good one. Another way to get better performance is to buy a new muffler. Sometimes the company will say it improves horsepower by 5, but really it will only improve it by 2 or 3. You can expect to get a good one for about 600$. Also getting a new muffler will make your car sound really good. Some ways to lower weight would be to remove your spare tire if you have one (but this is risky if your tire goes out on the highway or something!), and anything else you have sitting around in your car. If you have a spoiler, uninstall it. The wing will cause your car to be pushed against the ground. It is more for looks than performance.

I recomend these brands:

AEM

TANABE

TEIN

DC SPORTS

HKS

APEX I

GREDDY