It's the Castle in the Dwarven mines near the Underground Sea lift. I forget its name. But 100,000 is a ridiculous price for a rune.
I am having a similar problem as the OP, incidentally. In my first game as a Paladin, I do not recall any rune issues, but now as a Mage, I have about 16 Might and 17 Magic runes, but no Mind. At level 8.
