Where are you flying from/to? I thought it was $500 to upgrade TATL on US, but it could be less for PHL-DUB, possibly MAN/LHR. Keep in mind that there could be taxes on top of this.

I've been upgraded on US MR's a couple times...knew the Envoy FA, and he talked to the station manager about moving me to a vacant seat.
Another time I paid $500 to upgrade MUC-PHL. I had a bit too much to drink in MUC the night before, and felt barely able to stand, and the flight was fairly full. Then after I got back, I sent an email to CS about some seat issues, tray table, etc...and got a voucher for $250.

Generally going to Europe, I won't bother if on a MR. I'll take some Tylenol PM to knock myself out, and hope for the best.