My best was a comped junior suite cruise on Oasis of the Seas over Labor Day week last year. We upgraded it to a grand suite. Price paid for that cruise booking: $423 (fare, fees/taxes).
Royal Caribbean was giving double points, and we got double points for being in a suite, so we ended up getting quadruple points (28 for a 7-night cruise).
The casino offer I used also included $500 in freeplay. Due to unrelated things, the only time we spent in the casino was to use the freeplay. We ended up cashing out $540.
Net price for a week-long cruise in a grand suite earning 4x points: -$117.