"Don't know about the 770, but the 790 will run you, with FEL, around 15K and the hoe around 6K."
The 770 was the predecessor to the 790, so that one won't be listed on the JD site (hence, MachineFinder.com). The original question may have been a typo (should have been 790 rather then 770).
When I was looking to replace my 670 (smaller brethren of the 770 and 790), I looked at a couple 770's on line. They're priced quite comparably to the older 790's. Those 770's really hold their value! They're somewhat hard to find too. I guess the original buyers have no reason to sell 'em!