Everyone's going to have a different opinion on what's fair for a piece of gear. Those pants were going for ~$225 game worn from MeiGray if I remember correctly, but that's more of a collector's price. If it were me, I'd start at $200 and see where things go.
You could ask in the Caps sale thread what people paid for them at the recent sale, looks like he picked up a few pairs. If you're concerned about insulting them with a lowball and ruining your chances, send a message and ask. Based on their sold listings, they're willing to negotiate.
On topic: these would look nice for a Notre Dame setup
https://www.ebay.com...OUAAOSwrl5aw8hL