I replaced mine with a used aftermarket double-din touchscreen because I wanted USB/Bluetooth connectivity. Cost me nearly $400 in total including the radio, harness, adapters, dash kit, wires, connectors, etc. If you want to simply replace the knob, get a 3d printed one; much cheaper!
http://www.shapeways.com/product/4MVV6U ... ntiac-vibe
There are two methods to remove the old knob - take the entire radio out and apart, or crush the old knob to pieces and pull it out.