There is no standard. You might be offered something reasonable, it might be so high you just laugh, or it might be nothing at all. Have you tried to upgrade your flight online now? If you go to your reservations there's a tab for upgrade, and you'll see two displays one for miles and one for dollars. I've seen thousands of dollars, but was recently offered a buyup from C to F ORD-FRA for $550. It's like the lottery.