Is it possible to replace the tires and rims on a MazdaSpeed6 and put normal tires and rims on it?

Yes you can. You would need to make sure the new set equalled the overall height of the old set to keep the speedo correct. Any tire shop should be able to figure this out. If they can't, run away.

$100 for tires, about $100 ea for the wheels... mount and balance should come in around $900 or so.

Yes as long as you maintain the overal diameter of the tire. so if your rims are 18" with 225/50's (lets just say) and the 50 represents your low pro sidewall. you might need to go to a 16" rim to get a 60 sidewall on it. (but it wont handel as nicely, it'll feel real sloppy)

i should note that my numbers were just hypothetical. a tire shop will know what size you need to move to in order to maintain the same outer perimiter.

u can... no speedo problem it tires/wheels are same height...try tirerack.com or discount tire.com....get smaller wheels with tires that give same height,,,like a 14 or 15 inch wheel...