The cost of an Epic spell or power is based on the Spellcraft (or Psicraft) check necessary to cast/manifest it; each daily use of an Epic spell costs its "spell category base price" (for instance, for a use-activated spell with 50 charges, 1000 gp) × the DC. This cost is not modified by the artifact's caster level (since Epic spells and powers do not have level-dependent variables), but it is modified by mitigating factors of casting/manifesting the spell/power. An artifact with only a single use of any spell or power is destroyed when that spell or power is used, just as a potion is, despite being otherwise indestructible (see below). An artifact with 50 charges is not destroyed when those charges are used up, but instead recharges itself to the full 50 charges one year after the first charge was used (and remains as indestructible as a standard artifact until then).
An XP cost contributes a further 5 gp/XP just like normal spells and powers do.
If the spell or power requires multiple casters, then add the spell slots contributed times the artifact's caster level (for instance, an epic spell requiring four spellcasters to each contribute a 7th-level spell slot would add cost equivalent to a 28th-level spell at the artifact's caster level- even if that caster level is lower than 56th).
Finally, backlash damage costs an additional "base price" × the number of damage dice dealt.
Thus, an artifact containing the Hellball Epic spell (50 charges [==half base price] and use-activated [==2,000 gp base price], final base price 1,000 gp) costs 102,000 gp per daily use of Hellball it allows (Spellcraft DC of 90 = 90,000 gp, 400 XP cost = 2,000 gp, 10d6 backlash damage = 10,000 gp). Note that the item's caster level is irrelevant to this ability's cost, though it must be at least 21st since it is a Major Artifact. Also, this item loses its ability to cast Hellball for the year after 50 uses, though it only allows one charge per 102,000 gp in the base price to be used per day (and will keep any other abilities it has after the Hellball charges are used up, and will regain the 50 Hellball charges one year after the first one was used).
A caster-level 30 artifact containing the Verdigris Tsunami Epic spell (standard use-activation [==2,000 gp], no charge limit, base price 2,000 gp) costs 5,430,000 gp per daily use of the spell (Spellcraft DC 170 = 340,000 gp, 10,000 XP cost = 50,000 gp, 14 casters contributing 6th-level slots = 5,040,000 gp [14 casters × 6 levels × 30 caster levels × 2,000 gp base price]).
Finally, a single-use "gun" artifact (single-use, use-activated [==base price 50 gp], no space limitation [==multiply base cost by 2], final base price 100 gp) containing the Vengeful Gaze of God Epic spell has a base price of 61,900 gp (Spellcraft DC 419 = 41,900 gp, 200d6 backlash damage = 20,000 gp). It is destroyed after being used, though whatever it hits is likely to be as well given the 305d6 damage it takes.