I too think the huge jump in magic item prices is a pain, but I think I know why they did it.
First, what does WotC consider an appropriate item value for a given level? Looking at the NPC fighter, his main weapon generally costs the NPC equipment value for his level, subtracted from the NPC equipment value for the next level.
Let's take an average 20th level player character. He's got a BAB of +15, and a +5 weapon. That gives him an enhancement bonus of 1/3 his base attack bonus. For the past few levels, he's been able to purchase a +1 market modifier to his weapon every level. If this continues, by 30th level he'll have a BAB of +20 and a +15 weapon. That gives him an enhancement bonus of 3/4 his base attack bonus. Eventually, the weapon becomes more important than the character.
Now if we want to keep that 1/3 figure, we have two choices: decrease the ammount of money coming in or increase the cost of magic items. Decreasing the ammount of money makes very little sense. You're fighting more powerful creatures for less money? It is easier to rationalize increasing costs for epic items by saying they're harder to make because they are epic. IIRC, when you run the figures it turns out that an epic PC can buy a +1 enhancement bonus about every 6 levels. So after every 3 bumps to his BAB, he can get one bump to his weapon, maintaining the 1/3 figure.
Personally, I don't like this. As pointed out, you end up with epic characters with no epic items and a butt load of non-epic items. Why get a +8 to one stat when you can get a +6 to all of your stats for a third the cost? To me part of D&D is getting goodies. If you can't get new goodies that part of the game falls flat. I think it is evidence that the game as designed breaks down after a certain point. No big deal, these things happen. For my campaign I adjusted the item pricing so you still get lots of goodies, and put a level cap of 40 so that it doesn't get too far out of hand. After that you have to ascend. That give 20 normal levels, 20 epic levels, and 20 divine levels. It works for me. YMMV.