50% off PVC office chair mat for low to medium pile carpet $29.95 - alwaysdirect.com.au
I have to admit that I paid over $100 at Officeworks for a chair mat that does the same thing! Dont get me wrong, the thing has saved me from ripping my carpet to bits so it was worth spending the money but I wish I had looked around a little more and paid a fraction of what I did.
Anyhow, this one is on special and I would highly recommend you look at getting one if you like to see your carpet survive under your office chair. They also do them to protect hard floors such as wood and tiles.