[lucaspiller]

What are the costs exactly? Wifi operates in the 2.4Ghz band which is license free worldwide.

(As I understand in some countries you need to get a license to sell wifi devices - some Arduino's have issues with this - but not to use or build them).

[bcl]

You still need to meet the FCC specs for not causing interference (in the US at least), which can be costly. And you can be sure that a bare board with no shielding around the RF sections is going to be pretty noisy.

[Sanddancer]

I think the OP's talking about those who want to implement the boards on their own, where you do have to worry about all those certification costs.