Other points about DCs
Those incredibly hard DCs and XP costs are easier than you think...
Assuming a 20th level Cleric/9th level wizard, with max ranks in Spellcraft, and the Epic Spellcasting, Epic Skill Focus, and Craft Epic Rod feats.
First, said cleric makes a Rod of Epic Spellcasting. (122700 gp + 12454 xp). Next, he makes two (2) Rods of Excellent Magic (325000+26000 @).
At this point, his effective Spellcraft bonus is :
+32 (pure ranks)
+10 (Epic Skill Focus - Insight bonus)
+10 (Rod of Epic Spellcasting)
+30 (Misc. wonderous item giving +30 competence bonus to Spellcraft)
+ 3 (Int 17)
+ 3 (Headband of Intellect +6)
Total Bonus : 88
But why two Rods of Excellent Magic? This is why:
As a special use of the rod, the caster can substitute the power inherent in the rod for the experience point development cost of an epic spell. Doing so drains all the power from the rod, rendering it useless.
Enervate Kingdom
Evocation & Conjuration [ritual]
Spellcraft DC: 93
Components: V,S, DF
Casting Time: 6 minutes
Range: 300 ft.
Area of Effect: 20 mile radius
Duration: 25 hours
Saving Throw: Fort (Save DC: 60)
Spell Resistance: Yes (SR check at +20)
This spell affects a 20-mile radius, draining 10 levels from all creatures within the area for at least 25 hours if they fail the Fortitude saving throw. The level drain is permanent if they fail a second saving throw after 24 hours. The spell's affects are delayed by 10 rounds after completion of casting.
Base Seed : Energy (Weather)
Additional Seed: Heal (Inverted, Level Drain)
Base DC calc: 17+25+(17*2 [hours])+(4*6 [levels])+(9*4 [area])+(15 [ad hoc asjustment for delay])+(40 [dc bonus])+(20*2 [SR bonus])=17+25+34+24+36+15+40+40=231
Mitigating Factors: (20 * -1 [2000 xp to cast])+ (5 * -2 [add 5 minutes])+(12 * -9 [10th level wizards supplying 5th level slots])=-20-10-108=-138
Final Spellcraft DC: 93
Cost to Develop: 837,000 gp + 0 xp (would normally be 25480 xp)
Why the 10 round delay? Got give the Circle of 13 time enough to teleport away.
Total cost : 1609700 gp and 64454 xp, with all the xp cost (and about a third of the gp) for the rod creation.
And this leads to a Caution :
Beware Rods of Excellent Magic!
[Edit : fixed formatting]