The final round of bingo toward the end of the cruise is the only one that you have a real chance to win thousands of dollars. The "$10,000 Jackpot" bingos you'll see advertised mid-cruise only work if someone covers all within a limited number of calls, which I've never seen happen (and we always play Bingo). But someone always walks away with the final jackpot end of cruise.
So if you're looking for the best spend vs. most profitable winnings ratio, the final round is it. As I mentioned, though, we like to play as much as we can, just for fun. We never expect to win, but it's nice when we do (only small amounts, no jackpot).