The ELH does address this kind of question. One quote says something like, "If your players wanted to convert their wealth into gold, they'd have a metallic lump the size of a small moon." The point is that they can't convert it into gold; there isn't that much cash available in any normal economy. Epic characters' wealth consists of magic items and such.
A real life parallel might be Bill Gates. On paper he's worth more billions than I want to think about, but all that money is tied up in assets-- computers, cars, wives, the gigantic house, and of course a ton of stock. He doesn't have a Scrooge-McDuck-style "money pit" filled with cash; even if he wanted one, he probably couldn't get his hands on that many Sacajawea dollar coins.
As for spending on epic spells, all that wealth isn't spent as cash. Maybe you need a specific power component that you haven't time to retrieve personally; you can hire a lower-level adventuring party to get it for you, and reward them with powerful magic items. Perhaps developing your spell requires information from a diety, who will answer your question if you construct a new cathedral for his followers. The DM can play out this kind of thing if he wishes, but it's easier to assume that it happens during downtime.
