When people have managed to get refunds for preinstalled Windows licenses from major OEMs, the number's generally been in the $35-50 range.

Keep in mind that the manufacturers also subsidize the cost of systems with all the preinstalled crapware that comes on a lot of machines.

The OEM agreement prevents the exact numbers from being disclosed, but I believe it's common knowledge that it's quite small; probably well under USD50 for a company like Dell.