i did mine a whiel ago, do a search for wheel bearings for AE86 or something,
i got quoted 200 bucks a side (front) 100 bucks parts, 100 bucks labor.
i bought bearings, cones/races and grease seals for about 24 bucks a side, changed the bad side in an afternoon, anyone who knows a bit about working on cars should be able to do it.
i would have to assume that they would cost somewhere around this much for an MR2 also.
i didnt really notice it much driving, the rest of my suspension is loosish anyway, but you could rock the wheel and it was quite noticably bad.
the bad thing that will happen is that it can seize up and cause your car to do a one wheel handbrake turn.