I said this once on the podcast, but if games were priced relative to their development costs, you'd have to pay upwards of $100 for GTA, Zelda, Final Fantasy, etc. Also, I think it's wrong to assume that Wario Land cost less than a typical Wii game to develop. 2D animation is not cheap, especially at the level of quality in this game. When you think about simplicity of many other first-party Wii games, Wario Land could have easily cost as much as some of them.
Development costs aside, I do think it's natural in this day and age to question the value proposition of a 2D game, because we associate them with handheld platforms and less total playing time. With that said, I think it's completely possible that Wario Land is worth $50; it's just not something I can determine until playing it thoroughly. And I plan to do that as soon as my review copy shows up. (It's late, wah!)