General GMT II value question

[Sperry]

Hello, all. Here is a bad picture of a GMT II. The watch was purchased from an AD about six years ago,
so it would be a Y or F, I'm guessing. Never been serviced, has sat in a drawer for the last few years.
No box/papers, but has all the links. The original buyer is an established professional who has not budged
on the price for the two years I've been trying to get the watch for a few hundred dollars lower.

Nicely beat-up, chipped bezel insert, and has all the links. It's been a while, but I think it was a "holes" case.
Forgot whether it had solid end-links, however. Never been timed.

Keeping all this in mind, what do you think I should pay? Figuring the watch could use a $450 service
[or more] at any time. I'm eager to keep the acquisition price as low as possible,
even to the point of not spending a hundred more than the market might value this watch.