Won't start

I bought a 2000 Firebird 3 days ago, and it won't start. When I turn the key, the engine doesn't turn over, but it just sounds like there's a gear spinning freely. It's not the same sound as when your battery is dead. Could this be my starter? or a much larger problem? I'm sorry if this isn't descriptive enough....please help!

How is your voltage?. Does it make a clicking sound?. If it turns the starter but it isn't cranking the engine you may have a shredded starter gear.

no clicking sound...but how big of a problem is it to replace the starter gear? is it expensive?