I don't think it's the HD Textures and High poly models that make a game expensive as much as it the time and effort by programmers in order to optimize the engine and make it playable, correct me if I'm wrong here. Look, say I take a 7.2 megapixel picture of a brick wall, and I want to use that as my texture. It doesn't cost me less money to reduce that resolution to 1/4th the size, does it? What I'm saying is that developers go the extra mile in terms of production and optimization on PS3 and 360 AAA games that they simply don't bother with on the Wii, and THAT is what causes the game costs to be higher.
Take other technological features in HD games such as normal mapping, HDR, dynamic shadowing, etc. From here, you can take two sides. Either they are possible on the Wii at lower resolutions, or they're not because the system is too weak. If you take the former, you're saying it IS possible to squeeze those features in with effort. That effort would take a lot of time and money. The thing is, they don't HAVE to implement these features in PS3 and 360 games. They choose to because they CAN. And if it is at all technologically possible to put them on the Wii, they WOULD put the extra dollars of development cost and time in to make it work if they really wanted to. Any less would be use of the forbidden word "downgrade", and thereby a lesser experience. If, let's just say, the features were not possible on the Wii, and the developers were perfectly happy with that, then surely they would be perfectly happy leaving them out of the HD versions as well? If the argument is that the experience can be made just as enjoyable on the Wii, then the argument that the game would cost less to make on the Wii is moot, because producing that exact high quality experience is what costs money, regardless of the hardware that it is on.
That's about visuals. Any other aspects, and the whole argument is moot. Less sound? That would lessen the experience of something so story-driven. They didn't HAVE to put that much sound in the game, they chose to. Smaller maps? Lessens the experience gameplay-wise. People are making the argument that the exact same games could be put on the Wii, albeit a bit less pretty. If that's the case, you can't cut corners as such. Overall, I just don't see the hardware specs as the reason why the game development costs are different.