We did it in 2016 and went from a ocean view to veranda for $400 for a 7 night on the Magic. As stated above, it depends on availability and cruise and levels of upgrade. We got lucky on that Magic cruise. (Okay, I always feel lucky when I am on one of the ships)
We cruised 4 weeks ago on the Fantasy and there were no upgrades. My DW asked even though I told her we were not going to upgrade as we had chosen a different styled room and I wanted to see what it was like.