How much would you pay for gifting a watch to a woman?

Nothing. Nobody wears watches anymore, for one thing; everybody just looks at their phone. For another, a watch was traditionally more of a gift for a man, like at graduation or retirement.

Buy the lady diamonds is what I say! Earrings are good if you're friends, but necklaces are better if you want things to get sexy. Put it on for her and let your hands linger.

P.S. - If the woman has a self-proclaimed watch fetish, forget the above and spend at least a week's salary. Though the more expensive the better, assuming discriminating taste.