why will it increase in 2010?
Probably to generate good sale statistics. As when Vista was released the retail sale figures were used to show how well it was doing against XP release. That's what I read on this forum anyway. Also a good incentive to try and move the XP entrenched users.
Didn't find anything on Microsoft site, but here is a BBC News page that states £150 for Home Premium (retail) in January.