[Vicious Love]

Only one way to take a comment about being pedantic: Slowly and methodically.
Either way, I think it's fairly obvious why Caelum's mages are underpriced: They're a magocracy. They rely on mages more so than on mammoths, raiding capabilities, sneaky cold-dominion pushing Seraphines, and pretty much any other individual merit they possess as a nation(Though not the sum of all of them).
The mix of air and water magic also makes Caelum's High Seraphs SPECTACULAR battlemages, particular for hit and run raids. Cast quickness and three rounds worth of Thunderstrike/Orb Lighting, retreat, repeat. Support with antimissile amulets, robes of shadows, and a staff of storms for best long-term results.
Now, if Ulm were to get underpriced mages, I'd be seriously concerned, but since Caelum is one of the top 3 magical superpowers in the game, it's only natural for them to be cost-efficient.