There might be a way of remaining faithful to the idea of the ELH multiplier while maintaining affordability for low level characters.
I wonder if, instead of slapping a x10 modifier on epic items all at once, like the ELH does, it can be gradually applied. Starting from about x3, and then going up to x10 by level 40 or 60 or whatever. That is, price the item as if it were a non-epic item, then multiply by an epic multiplier that scales smoothly to x10 when you get to high-epic levels.
Since there are different ways that an item can “go epic”, the formula might be complicated. For instance, the epic multiplier might be (half epic enhancement bonus, rounded down) or (1 + amount total bonus exceeds 10), whichever is higher. This multiplier is capped at 10.
So a +6 enhancement bonus on a sword will cost 216,000 gp (triple what the non-epic formula would yield). A +5 sword with +6 in special abilities will cost 484,000 gp (double the non-epic formula). A +6 sword with +5 in special abilities would cost 726,000 gp (triple the non-epic formula).
Items like armor, shields, and resistance bonus items cost half as much as a weapon with the same bonus. A +6 shield would cost 108,000 gp in this system, but I don't think that would be a problem. It's still more expensive than a +5 shield with 5 pluses worth of special abilities, and there's no reason why someone would buy a +6 shield rather than invest that cash into a slotless item that gives a +1 insight bonus into AC or something.
Anyway, with these prices I think Charlie will have to be 31st level to afford that sword. He’ll have 2,979,100 gp, and his best item could cost 744,775. E.g.
+6 speed adamantine bastard sword (729,335) (#1) (multiplier = 3)
+8 mithral heavy fortification heavy shield (677,020) (#1) (multiplier = 4)
+11 mithral chain shirt (606,100)[/i] (#1) (multiplier = 5)
Charlie’s AC and attack bonus is a little less, but he has 151,145 left to spend; he can surely make it up somehow.
This formula be tinkered with, of course. But I don't see why we have to have a cubic + constant if something playable could be derived that will mesh with the equipment in the ELH and UK's work.
[I would suggest that we revisit the [augment] seed to see gain a clearer perspective, here].
If we think that [polymorph] is OK, then it might provide some insight into buffs. [Polymorph] won't give much insight into the problem of high save DCs, but if everything falls into line, perhaps it will too.
But as far as equipment goes, I seem to recall that the item buffs were acceptable using the ELH formula. Since my suggestion converges on that formula, it might be OK too; an extra +2 DC or so won't break the system.