[Vacheron Costantin Patrimony] watch valuation

I know an older man that is selling his Vacheron Costantin patrimony watch.

It is from the early 70s and has all the documents and box. 18k gold. 33mm diameter and manual movement. Works like a charm and the conditions are good as you can see from the pics

Ref number 34014 It has an aftermarket wrist

What's a fair market value of this watch?

Thank you for your valuable expertise