I would say that spell level in D&D is roughly equivalent to spell magnitude in Runequest. I've been using that theory in evaluating Runequest casting mechanisms.
One difference is that in converting a D&D spell to Runequest, I'd consider reducing the damage done by a high level spell.
Runequest provides a much more flexible magic system, in that divine, sorcery, and runecasting are three very different systems, while in D&D all spellcasting systems are either based on the Wizard or the Psion. This makes it harder to evaluate balance because you have to carefully consider the way this will play out in Runequest, as opposed to just saying "Okay, my Judge prestige class maxes out at 5th levels spells, which makes them a little better than Rangers but not as good as Bards."
