The volumes of cash that Microsoft throw at retailers (custom builders / big box) is astronomical. Worked for a relatively small retailer with some international buying power. EOFY "MDF" from Microsoft was an absurd figure.
Our builders would belt out 3 - 6 machines per day, depending on complexity of the custom build, the pre-built machines were in the 6+ per day range.
Considering the vast majority of those machines were running windows (some sold without an os), from a quick estimate after too many beers we were out of pocket 10% at most of the bulk buy price for licence keys after our "market development funds" came through.