I almost never see them when I book V fares. My guess is that they are more common on the higher fare classes. It would be helpful to know OP's and companion's original booking classes and if they were the same. Without that, we are just left speculating. Someone could do something crazy and start a wiki here to document them (original fare class, upgrade price, upgrade fare class, etc.).


Okay, I just got lucky and found one for one of my future itin's in Feb. Original booking class and price: V fare, $105.60. Uprgrade offer $554.00. Current pricing for F on same flights - $659.60 (H-UP fare). In other words, the upgrade price is the exact difference between what I paid and the price for a brand new F ticket. Unfortunately, the fare rules link on my upgrade offer don't show any details. But I think's it safe to assume it's just the F fare (HATNA0FE) I see when I go to book a new ticket given that the price diff matches.