@Maddus Mattus: unlikely, IIRC the reason they make it an optional pack is to save the decoder licensing cost for those dont use it. if its free, where is the cost ? why not build it in, out of box ? this doesnt make sense.
Pricing for these Packs, as well as retail versions of Windows 8, will be announced closer to the release date. To give you some indication of Media Center Pack pricing, it will be in line with marginal costs.
Maybe these 'upgrade' packs for XP/Vista/7 users are the same as the 'Windows 8 Pro Pack' for 'Windows 8' users, which contains WMC. Maybe its just part of the deal till Jan 2013. Who knows, its alwasys confusing.