I have a player that wants a pair of Mithril pants...
Granted there is nothing in the DMG stating there is a thing.
But if there were would you allow them? Woould you give them a modifier to abilities?
I think someone has been playing too much Baldur's Gate II. Just pokin'. Just charge them for a mithral chain shirt and grant them a really small AC bonus, +1. The reason is because a chain shirt grants +4 to AC, while chainmail, which is basically pants of chain with a chain shirt, grants only a +5 to AC. So the addition of the pants doesn't really make that much of a difference. I'd just charge 1,005gp (1,000gp for the mithral quality and 5 gp for the chain pants themselves, or something like that.