There really is no simple answer because (on most platforms) the low budget games for a platform can cost less than 1/4 the cost of a big budget game; and companies that were prepared for the switch to the HD platforms have offshored enough of their content creation to keep development costs reasonably flat.
In general though, it has been mentioned by (pretty much) every major publisher that the development of a similar game under similar circumstances costs 3 to 4 times as much on the HD consoles as it does for the Wii (or previous generation consoles); and the development cost of a Nintendo DS game would be in the range of 1/4 to 1/2 the development cost of a similar Wii game.
After playing through MadWorld and watching the credits it looked like there were (roughly) 20 full time employees and I believe the game was in development for 18 months, and there was (roughly) 10 to 20 part time employees and short term contractors working on the project. As a guess I would put the development cost of the game in the $4 to $5 Million range. The same can be said of many of the Wii's "Bigger" third party titles this generation, and most of these games would fall in the middle of the range of previous generation development costs.
While it is difficult to get good estimates on the development teams of all HD games, it seems most "average" games seem to have development teams of 60 to 80 people working on a project for 2 to 3 years which puts the development costs in the $15 to $30 Million range. At the high end of the super anticipated games there have been projects with (far) more than 100 people working longer than 4 years to complete, and these games are approaching development budgets in the $50 to $100 Million range.
