Paid Upgrade from reward PE ticket to UC?

I've got a reward ticket in Premium Economy and was wondering if I can pay for an upgrade to Upper Class for one of the flights? If I can does anyone have an idea how much the upgrade might cost?

[mitchja]

I'm afraid you can only upgrade reward fares using more miles.

If you haven't got enough miles you could of course purchase the extra miles needed (provided there's award space available in UC that is!)

UNLESS -there is a rule -anything can happen at the airport. I have upgraded REW tickets before but it is pure pot luck and down the the ticketing staff. cost -anyones guess

Thanks for the answers I guess all I can do is ask at the airport and hope for the best.