I've often wondered this myself. The minayn is part of the service that Kosherica provides. I don't buy the high holidays service argument because whether someone pays or not for the high holidays is often dictated by ability and/or affiliation with religion. We should not turn away someone from davening because they can't afford it or even if they can, because they don't feel close enough. A cruise is a luxury to start with and in essence, we are deciding to not pay the extra money because it is not worth it.
That being said, I would highly doubt that if Kosherica was just about minyan, that anyone would pay any more than a $100 bucks to ensure a room and sefer torah. I don't see how the minyan portion of Kosherica is anything but a service to make sure that they don't lose customers who insist on minyan. The extra expense a Kosherica consumer is paying is for the high level Kosher food and entertainment. Some people want gourmet kosher and jewish entertainment and are willing to pay for it. If there was a woman's only kosher cruise and no minyan, I doubt the cost would be much lower. I don't think many (there might be a small percentage) people are going on Kosherica, as opposed to other vacation options, because minyan is expected. it is more to ensure that people don't nix the idea and I would think that if a somehow a Kosherica group went forward where they only managed 8 men, they would be thrilled to get the help of a few more to complete the minyan. I would hope that they would welcome people to the minyan