I already thought plague of toads was an odd spell to have as a signature spell as it's really the only one which you can't generate resource while casting it for quite some time. But the question is, why is the level so high? Honestly, I figured when you've reached a level like 54, the mana cost for a signature spell should be irrelevant, as you would making plenty of it back anyways. I would accept if this was a level 30-35 rune, but I honestly think that it's a waste of a rune at the current level.
None of the other classes have a skill like this, in fact, most classes the signature spells are all resource generators or cost nothing. I can see it's use in having the ray of frost spell for the wizard manaless, but it that secondary skill, not a signature skill. I'm just confused why you would need a signature spell, which are basically built around being a skill you can use while you're out of mana, would need such a rune. On top of that, it's counter productive by unlocking it at such a high level in which the significance of having such a rune is even worse.