I think it is unreasonable to expect the game to cost the same for Sega as it sequel costs for Nintendo. Besides progress in technology, and reuse of assets (such as planning) Nintendo is a far more conservative fiscal company. Furthermore, Nintendo is pleased by much less than Sega might be. So 500k-750k could be a success to Nintendo if that means a profit.

they want to win over enough of those people who are looking for distinctive games which they would be willing to buy a wii u for as a secondary system. nintendo games are always the selling point for nintendo hardware, even with the nes which had tons of third party support... it was, what, atari games vs. nintendo games, and the atari 5200/7800 lost big time when that was the proposition in the west... anyway, my point is bayonetta fits right in with that proposition--unique games you can't play elsewhere, and in a way it diversifies their offerings by adding some mature games to the mix. where else can you play wonderful 101, bayonetta 2, tropical freeze, captain toad, 3d world, smash, etc.? i think ultimately differentiation is always nintendo's goal. one of their failures in the wii's post-2010 decline was to make that system vibrant in its last few years. they had galaxy 2 and skyward sword, two of the best wii games, but not enough of that weird, unique (you might say niche) nintendo content that makes nintendo hardware compelling.

Man, I would love it. I haven't even played Bayo 2 yet, still on the first one. 30 hours in. so addicting.

Nintendo should absolutely be willing to lose money on a few niche titles in order to diversify their offerings. 500k is a good number of people, profitable or not. I dunno if they'll make a sequel, but with all the critical acclaim being piled on Bayonetta 2, I think they should definitely go through with it. Immediate profits shouldn't be at the top of Ninty's list at this point.

Maybe depends on which company wants it most

I can see Nintendo funding the third game, they need partnerships and titles like this to diversify their library.

I can also see them funding ZombiU 2 as well for the same reason.

Sales of Bayo 2 don't matter it all comes down to if Nintendo wants to fund it. The only reason they would is in the hope that they have enough mature exclusives (as one isn't enough for most people) to entice some Wii U purchases from core games which may lead to sales of their own games. They will not profit from it though so they have to be prepared to lose money in the hope to make more money elsewhere.

Saying all that they will need at least 10 or so AAA mature exclusives to entice people.

We can assume a few things. If Platinum wants to make a third they'll approach Nintendo first. So it's more likely the 3rd will be on Wii U.