Naive question on casting 500+ fatigue spells

[Graeme Dice]

For a better explanation, Solar Brilliance is an astral 5 spell that requires 5 astral pearls to cast (and hence requires 500 fatigue to cast). I have personally seen astral 4, and astral 5 mages cast this spell against a horde of undead without going over 200 fatigue. This is only possible if they are capable of spending the 5 gems as the base cost, then up to 4 or 5 gems more to boost their power level and reduce fatigue.