I have a 74 inch 917H, and would love to help. But there's no way to put a value of that machine on the basis of those 3 photos. I'd have to check for slop in the bearings, see if the shafts and roller were true, evaluate wear on the roller bar and skids, condition of knives or hammers, et cetera.
What I can do is relate my own used flail experience. My 917H cost $150 at an implement auction. It was sitting on the ground so there was no way to test the bearings and bushings. Didn't have a PTO shaft or belts anyway. Some of the knives were missing, and at least 75% of them needed replacing. The knife hangers were all at least 50% grooved and in need of replacement too. But at $150 I thought it was worth the gamble to take it home, mount it on a tractor, and give the shafts and bearings the test of fire. I spun it with a temporary PTO shaft, and - to my relief - everything was straight and solid.
So that meant - to me - that it was ok to spend more money on it. After a new PTO shaft, knives/hangers/hardware, straightening the skids, replacing a few missing nuts and bolts, a new banded belt - I had an almost new/old 917H. It still needs a paint job, but at the moment I think I've got ~$600 invested. Once I get it blasted and painted, I'm speculating that it might bring upwards of $1000 from someone who actually knows what a flail mower is. I say that, because around here hardly anybody outside the highway department uses them, so the term is not used very often.
//greg//