It's worth $20 if it's in decent shape.
I had an XR-7 I bought in college shortly after they came out and used, sometimes heavily, sometimes (like the previous 10 years) not at all, until last year when the rewind knob broke. The rest of the camera is still fine (I did have the light seals replaced once.) I bought another off eBay for $25 with a junk lens. It's my second body and I do use it a fair amount.
In my defense I'll add that my primary 35mm body is a Pentax LX. ;-) Still, while it's a plastic toy in comparison to the solid and well built Pentax, it is nice to use and has some good features.