My rear tires are going bald on the "inner" side; the outer side are ok???
what has caused this???? I am going to get new tires, but like I said, what has caused this; is this going to cost me alot to fix??? I have a 1999 Pontiac Grand am.

Welcome to AF !

Most likly an alignment issue, if the tire pressure is correct.
Alignment runs anywhere from $50-90, depending on what all needs adjusted and your geographical area.

I'd get it checked out though. The tire shop should be able to tell you if anythings not right back there. And computer alignment is a must.

Hope that helps!