Cost per wear is the purchase price divided by the number of days you've worn the watch. A 6,000 watch worn on 120 days has cost 50 per wear; worn on 300 days, 20. It works in any currency.
Why collectors track it
It makes wrist time visible. Two watches bought for the same price can end up worlds apart: one worn three days a week, the other a few times a year. Cost per wear points to the pieces earning their place, and to the ones you might enjoy more by wearing them more often, or by letting someone else enjoy them.
What it doesn't tell you
It ignores what the watch is worth now. A watch that holds its value has cost you less than its price suggests, and services add to what you've spent. Read cost per wear alongside an estimate of the current value, not instead of it.
It also falls with every wear, so a new watch always starts high. Give it a few months before comparing it with pieces you've owned for years.