One of the other problems with magic items is that different games have very different systems for how magic items are acquired. (This is an "other problem" in the sense that it's not about how many magic items are necessary for a balance level.)
I wonder if the hypothetical 5e would be better listing creation costs rather than market prices. Creation costs could be used for PCs making items and as a neutral cost basis for PCs creating characters above first level. (If rarity is also a characteristic of items, a PC creating a 7th level character might have a total budget with which to buy 3 uncommons and unlimited commons with a max level of 9.) Creation costs would be based on the power of the item and could be relatively consistent between campaigns.
The market price for the item (i.e. the cost of buying or selling items), on the other hand, could have several variations, depending on the type of campaign. For example, a GM could choose a "free exchange" system where items are available for cost at an appropriate venue and can be sold for 20%, 50% or 100% cost, depending on rarity or GM preference. Or, a GM could choose a no magical trade system where magic items are only available for sale or purchase from particular NPCs. Or, a GM could place "markets" where particular items are for sale. A major regional center in a heroic game might have common items of up to level 12 available for 110% cost and a 20% chance of having any given uncommon item up to level 9 available for 150% cost.
By publishing a list of the creation cost instead of the market price, WotC would be providing the underlying power information and letting the GM decide how to convert that number into a magic item economy appropriate for his game.
In any case, it should be different from how it is presented in 4e, where the "market price" isn't really the market price. It's the purchase price if you ignore the (somewhat arbitrary) guidelines about purchasing magic items. Who needs that?
-KS