They are pretty rare. $20 is a very good price even if it doesn't work and is in rough but restorable condition. I've gotten decent looking ones at swapmeets for $5-10 but people selling them had no idea what they were from. If seller know what's he selling they'll usually be asking ~$100 for them. I paid $200 for a completely restored one about 15 years ago. 5-6 years ago even really rough ones were selling on eBay for $100+. The last one I sold on eBay, about a year ago, went for $50. It do.didn't work but was in good condition.
Unless you really want a working original radio I'd suggest you clean up the truck radio and use it to fill the hole in the dash. You can run I-Pod or some other electronic music device to play music. You can also have a place replace the original guts with new electronics to make it play AM/FM/USB device.