I've been pondering how a designer would approach pricing magic items, and there are a few issues I've come up with.
1) You need wealth by level guidelines. Otherwise a great (or poor) roll on a hoard could unexpectedly unbalance a campaign. If you have no idea how much gold will be floating around in your campaign, how can you set fixed prices? Additionally, you want to leave only a minimum of spare gold (intended to be used for anything but magic items) or the players may use the extra to buy more magic items than the guidelines intend. (I have this picture in my head of an epic level party decked out in a king's ransom of magical bling but dressed in rags, living in the woods to avoid having to pay for an inn just so they can afford their next big upgrade a little sooner.)
2) There's a fine balance to be had. On the one hand, if the prices scale too slowly, players can forgo buying lower level items in order to get a really powerful item much sooner than they ought to. They could even pool their resources to buy an extremely powerful item.
Conversely, if you make prices scale too quickly, players can forgo their next + 1 in order to buy a near infinite supply of lower level items. (The best example I can think of is the purchasing of multiple wands of Cure Light Wounds for minimal sums in 3.x, which made healing between combats functionally unlimited.) This is the method that 3.x and 4e used, and is in fairness probably the lesser of two evils.
3) There's no way to price items in a way that is fair to all characters. A cloak of displacement will be far more powerful in the hands of a fighter with optimized AC, than on an unarmored wizard with a poor dexterity score. The two best options as I see them are to either price for an optimized character (which means that characters who aren't optimized in that respect probably won't bother), or find an average price somewhere in the middle (in which case there's little chance that the optimized character will pass it up).
None of the above is to say that it can't be done. However, I don't think it's possible to come up with a system that finds an ideal balance. Even if it's done by WotC and they really take their time to get it right, I think it will be exploitable.