I'm seeing so far it's much more costly to protect against a harm spell of roughly equal effect.
Correct. However, one protection spell can protect against multiple attack spells.
I've not yet written down the rules, but my thoughts go roughly like this:
Each level of Effect in a protective spell, will reduce the Effect of attack spells by one. So an Effect-1 spell would be enough to reduce a spell like "There Can be only One" to non-lethal.
However, each attack with an equal or higher Effect would also reduce the Effect of the protective spell by 1. So the above example, that protection would be cheap (1-2-2 = PL 4) but only prevent 1 attack, and you'd still be seriously injured. But the really powerful defenses (say, 5-2-3 - PL 30) would last you quite a bit - all attacks would be blocked completely, and only Effect 5 attacks would reduce its strength.