From the 'sticky' at the top of the forum:
How much will the new HD channels cost?
DirecTV HD access for non-premium channels is currently $9.99 per month. Premium HD access (HBO, Showtime, Starz!, etc.) is included with premium channel subscription. A new HD programming package, the 'HD Extra Pack' contains HDNet Movies, MHD, Universal HD, The Smithsonian Channel HD, and MGM HD. It's currently free as part of the new HD rollout, but after 12/15/07 the cost is an additional $4.99 per month. The reason given is these channels do not have an SD counterpart. The latest update (12/7/07) is that HDNet and DirecTV have resolved their law suit over the HD Extra Pack. HDNet will remain in the standard HD tier and HDNet Movies will now be included in the HD Extra Pack. Details can be found here: http://www.avsforum.com/avs-vb/showthread.php?t=935780