How does one balance a PrC?

[DonAdam]

Can you say minmax? I knew you could.

It is a min/max, but that's more a problem with the archmage. The requirements make it anything but "arch-." I don't see many high level wizards that aren't uber-specialized not taking it.

 

[Voadam]

I'm not sure about the epic Mystic Theurge progression to be honest.

Unfortunately the SRD does not cover individual prestige classes, but it does provide guidelines:

EPIC CLASS FEATURES
Many, but not all, class features continue to accumulate after 20th level. The following guidelines describe how the epic class progressions work.
• Class-related base save bonuses and base attack bonus don’t increase after 20th level. Thus, these class tables have no columns for base save bonuses or base attack bonus. Instead, use Table: Epic Save and Epic Attack Bonuses to determine the character’s epic bonus on saving throws and attacks.
• A character continues to gain Hit Dice and skill points as normal beyond 20th level.
• Generally speaking, any class feature that uses the character’s class level as part of a mathematical formula continues to increase using the character’s class level in the formula.
• Any prestige class feature that calculates a save DC using the class level should add only half the character’s class levels above 10th.
• For spellcasters, caster level continues to increase after 20th level. However, a character’s spells per day don’t increase after 20th level.
• The powers of familiars, special mounts, and fiendish servants continue to increase as their masters gain levels, if they’re based on a formula that includes the caster’s level.
• Any class features that increase or accumulate as part of a repeated pattern also continue to increase or accumulate after 20th level at the same rate. An exception to this rule is any bonus feat progression granted as a class feature. If a character gets bonus feats as part of a class feature these do not increase with epic levels. Instead, these classes get a new bonus feat progression (described in each class summary below).
• In addition to the class features retained from lower levels, each class gains a bonus feat every two, three, four, or five levels after 20th. This benefit augments each class’s progression of class features, because not all classes otherwise improve class features after 20th level. These bonus feats are in addition to the feats that every character gets from level advancement.
• A character doesn’t gain any new class features beyond 20th level. Class features with a progression that slows or stops before 20th level and features that have a limited list of options do not improve as a character attains epic level.
Likewise, class features that are gained only at a single level do not improve.

From the above I believe they would continue 1/1 in both wizard and cleric caster level but get a poor bonus feat progression.

I have been told, however, that it is spelled out in the DMG and they alternate gaining in class levels and get poor feat advancement.