The cost of an Epic spell or power is based on the Spellcraft (or Psicraft) check necessary to cast/manifest it; each daily use 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 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 and use-activated, base price 1000 gp) costs 102000 gp per daily use of Hellball it allows (Spellcraft DC of 90 = 90000 gp, 400 XP cost = 2000 gp, 10d6 backlash damage = 10000 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 102000 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, no charge limit, base price 2000 gp) costs 5430000 gp per daily use of the spell (Spellcraft DC 170 = 340000 gp, 10000 XP cost = 50000 gp, 14 casters contributing 6th-level slots = 5040000 gp [14 casters × 6 levels × 30 caster levels × 2000 gp base price]). Finally, a single-use artifact (base price 100 gp) containing the Vengeful Gaze of God Epic spell has a base price of 61900 gp (Spellcraft DC 419 = 41900 gp, 200d6 backlash damage = 20000 gp). It is destroyed after being used, though whatever it hits is likely to be as well given the 305d6 damage it takes.
