What? The $15 - $25 the OEM computer manufacturer pays to Microsoft for Windows? That is hardly a "monopoly" price.
Ten years ago the cost of a copy of Windows was about $100 retail. The cost today? The cheapest that I have found a full copy of any supported version of Windows is around $120. Now I'm sure that the price is lower in large quantities, but that was true then too.
This is the cost of a monopoly. A monopoly has no market to push prices lower. A monopoly has no market to force higher quality software.
The perfect example of how a monopoly costs is the cost of a minute of long distance in 1975 and the cost today. Even accounting for inflation, it's lots cheaper today. And no, local calls don't count, the Baby Bells still have local monopolies on local calls.
Whether it's retail or through OEMs, Microsoft is gouging the market with it's monopoly.