These can sell for $200. I know this because I've seen ones with "buy it now" sell at that price.
Failure points to look for in an SX-64:
Distorted sound
Missing keyboard cable
Disk drive failure
handle damage (and like Tj said, missing blue caps on the handle)
There are a lot of common SX-64 issues that you will see even with the "mint condition" ones you see on ebay.
What I ended up doing is combining parts from 2 SX-64 and 1 c-64 to make a complete, good-condition SX. I think that's the cheap way to go if you are willing to do a little restoration work.
I'm a SX-64 lover if only becuse it keeps me from needing to add a 1702 monitor, C64 console, and 1541 drive to my collection.
Just my .02
