I think it depends on which ship and which itinerary. I dont recall the price but we upgraded from a 1 BR to the Roy suite on the Wonder a few years ago but it was only a 3 night cruise so it wasnt that much compared to say a 7 night eastern on the Fantasy.
At least as of Wednesday, this week's Fantasy sailing on Sep 14 had the Walt Disney Suite vacant. Unless someone paid full price in the past 3 days, someone is going to take advantage of a great port upgrade this Saturday.
About a year ago the upgrade on the 7 night Fantasy cruise from a Cat 4 to the Cat R was just over $10,000. About $4,000 from the Cat 4 to the 1 Br. So an estimate from the 1Br to the Royal would be between $4-5,000 and that is in line with what I have heard reported.
We were watching the one open Cat R on a January cruise since it was still open. Talked to the hotel director during the cruise and someone grabbed it at 11:35 the night before the cruise. So people will still make the switch last minute and pay full price.