Can a Blood Pact let me spend Power for spells instead of Sanity?

No: the pacts designate the ability to stop losses , which are distinct from costs .

However, when you make a pact of one kind for the first time you CAN ignore the Sanity costs, so basically that means you get to cast three spells successfully and for free three times. Of course no one would EVER want to make THAT many pacts, right?