.
The mana costs seem to have been roughly based on the spells from the online game in relation to one another. Things that cost a ton of mana online also cost a ton in the rpg.
This works for most spells, but some have been altered to better represent the cost. The Alkabor spells, for instance, are complete and utter trash (possibly the worst online spells of all), but they're pretty darn useful in the rpg.
Ps. Flaming Sword of Xuzl just has more character than Mordenkainen's Sword, no? =)
Heylel