[ericbsmith]

What you're missing is that most spells with high energy costs are either going to be cast Ceremonially (see p. 12 of GURPS Magic) or as an Enchantment (see Chapter 2 of GURPS Magic, starting on p. 16).

Ceremonial magic is cast by a group of mages who can each contribute energy into the casting, including using Powerstones to contribute some energy. This can bring the total available energy somewhere in the neighborhood of 100 energy without getting into "industrializing" magic.

Enchantments are kind of a special case of Ceremonial magic, where one or more enchanters can contribute one energy each per day of enchanting until the spell is cast. So a 4-mage circle casting a 400 energy spell will take 100 days to cast it.