General cruise nugget:

Seascanner.com for seeing availability on your ship and judging price drops or upgrades.

I just finished a 7 day on the ncl Jade and paid for an upgrade onboard the ship from Oceanview Cabin to Penthouse with Butler cabin#9500 and what must have been a 300sqft balcony). Cost of upgrade for 2 pax $1390 all in.

If the ship has a lot of suites (see link) your chances of scoring this onboard, on NCL at least is pretty good.

Reckon that cabin was selling at about 6k the day before the cruise.