What does the cost of dev kits have to do with whether 3rd party developers want to develop for Wii? Dev Kits for Gamecube and N64 were cheap as well, but they had very little 3rd party support. $2-10k to spend on a dev kit is nothing for a developer. That has never been the issue.
The fact is, the vast majority of 3rd party titles are using 360/PS3 as their lead platform, and most of their games won't be ported to Wii because Wii is way too underpowered in comparison to be able to use the same game assets used for the 360/PS3 versions.
Wii's lack of power is the main reason it will have a very limited library of games. Developers simply aren't going to port their premier titles for it.
PS3 and 360 will have a library at least 5X the size of Wii.