What you do is simply say that artifacts are merely physical extensions of a beings own inherant power. So they grow in power as the character does.
So you're using the term "artifact" to mean "epic magic item". That's kind of misleading, given that "artifacts" have, for 30 years, been something totally different wouldn't you say?
I do institute such a rule.
Right, but how does that work in play? Do the PCs' items just automatically increase in power as the PC gains levels, without the PC having to spend gold and time to upgrade them?
On the level 20 PC affording a +20 weapon: Well, look at it logically - Where will he buy it? Who would sell it?
Yeah, I neglected to mention that he might be able to
afford it, but he wouldn't be able to
buy it.
Using the wealth by city populace guide, even in the largest cities or metropolii, only non epic items less than 200k would be available.
Ignoring for the moment that their "upper limit" of 25,000 people is utterly absurd*, you're right.
*Some medieval and even pre-Christian cities had several hundred thousand or even a million people in them. Waterdeep has a milliion; so does that city in Sembia whose name escapes me, and I'm sure there are several in Eberron too. If you extrapolate an extra 100K gp item value per 25K population, you'd get around 4 million gp limit for a million people so it's entirely possible that in a place like Waterdeep, you'd be able to find a +6 weapon.
Buying a +6 sword is like buying the Sword of Kas. No one would ever sell it. And they probably wouldn't tell anyone about it.
True, it likely wouldn't ever be sold, but.. I don't know about your world, but in ours, people use their magic items. Of course, ours is a world based around war, where you use every advantage you can get, but I couldn't really see a +6 item (weapon OR armor) sitting in someone's treasury gathering dust unless: a) it's a (religious) artifact that belonged to a hero of the order; or b) it's buried with the person who used it. If I'm powerful enough to have such an item, I'd most likely be using it.
Why would a mage make a magic item: If he is helping his fighter buddy, he probably makes one good item for him. Thats it. Its not worth it to make too much more. (xp hurts at higher levels)
Don't get me started on the ridiculousness of epic pricing and XP costs for magic items. I'll just give you two links and leave it at that:
Epic Item Progression (cheaper XP costs) and
artificing (no XP costs).
A very high level (not even necessarily epic) character might not even need silly expendable items. By the time he can afford them without effort. (Example: Expensive Wand of Stoneskin) The effect is easily availible to the mage in the party, and it's effect is too weak to matter (foes will be high enough level to afford adamantine weapons, or be potent spellcasters).
True... they'd be buying higher-level potions (elixirs, or whatever) like potions of heal instead of potions of cure serious wounds. Lower-level potions are still handy for when the party spellcaster is out of spells, or when the party member in question is alone (cut off, scouting, whatever), but yeah... those situations would be fairly rare, which means he wouldn't have a lot of those.