Eleven years ago the average desktop PC was priced at $1,926 and the average notebook at $2,819. Nowadays it's respectively $801 and $1,256.
As an additional comparison, average prices for desktop computer -- monitor included -- and portable PCs have been noted for all releases except Windows 3.0. (Data provided by IDC.)Check it out over here.
While these "average" prices don't spec out systems identical to Microsoft's minimum requirements, they are a good indicator of what users paid for machines to run each OS.