I am not sure we will see lower prices. Firstly, prices in Apple's store already are, let's say, 'affordable' for most apps.
Secondly, I think many apps are priced to what the market will bear, not to recoup costs plus some profit.
Thirdly, running that store will cost MS money. They are not a charity, so I guess they will try and recover the costs in some way. What that way is, I do not know, but I would guess it would be in adverts, non-free options with better exposure and/or in the cost of Windows.
