Windows costs the OEM much less, it's almost free when you add in the money they get from bundling software like antivirus trials etc.which they won't get from installing Linux or selling without an OS.

Also their support staff are trained to handle Windows. Adding Linux to the mix will increase costs.

What would happen is the Asus Eee, which got such a high rate of returned units from people who didn't understand Linux that it ultimately got canceled.

I wonder what's the rate with current Chromebooks for example?

The UX of the distro chosen for EEE PCs (unfortunately I don't remember the name anymore) was really strange and something that was alien for users of any mainstream Windows, Linux or Mac desktop. I wonder if it could have succeeded better with something like Xubuntu instead.

Most likely, the price would drop no more than 10$. OEMs don't pay much to Microsoft.