I have both. I can do $25 shipped for the 645mg, or $40 shipped for the Z650M. The 645 is in very good condition. The 650 is new out of the package. It's been sitting in my pit box as a backup.
Might I humbly suggest you give the Tower Pro MG995 a shot? They are cheap and tough as long as you get your endpoints setup right. If it doesn't work out and you break it then at the very least you didn't pay a lot for it
I've used them on my Axial crawler (which gets a beating on the steering) and on my WK basher and under both conditions they have held up just fine.