suprisingly no, they are not expensive to fix. now serious performance parts (not ebay specials) can get extremely expensive for small power gains. but when you have a 300hp mr2 and you can get into the 11's easy, it can be worth it. And yes, you can hit 300hp on stock internals.
now yes, they are more expensive when you compare them too civic and del sol parts, but they are not too bad.
as far as mainting a 91 or so mr2, most people have to deal with struts, ignition parts (plugs and wires), and in alot cases of high mileage, the turbo. which all of these can be changed with about 700 bucks.
i can go on for hours with this