Join Date: Feb 2000
Location: San Francisco (Marin County), CA
Programs: UA Global Services, AA Lifetime ExecPlat, LH HON Circle, SQ PPS Solitaire, Hyatt Courtesy Card
Posts: 3,002
I have been through this several times and was always charged 10% of the price for the cabin in which we actually flew, regardless of how we got into that cabin (upgrade, full award, fully paid). I think 10% of the F fare is the correct implementation of the rule here.
By the way, this is the same result on all other carriers we have traveled on with lap infants in business or first (which has been a lot of carriers for us).