$50/hr, perhaps a bit higher for a good quality business class product that includes lounge access prior to departure (such as AC). For Ewwnited, not very much.
Also depends on Aircraft type and load factor, if its empty in the back I would be less likely to pay the extra $$. I find AC's E90s to be very comfortable in coach, so I would be less likely to upgrade vs on an A320 with 3-3 seating.