I very much like the idea. One worry is that the spells are too varied and unpredictable to build a deck around. I see four cards to compare it with: Tournament, Black Market, Knights, Ruins
Tournament: Here, you get to choose the Prize.
Black Market: You get three cards to choose from. If you still don't like them, at least you have +$2 for your buy phase.
Knights: Each Knight has a common effect which is its primary, reliable effect. The additional bonus is comparatively minor.
Ruins: These are junk cards anyhow, so the variation in effect is largely moot.
With these comparisons in mind, perhaps all spells could be available, like Prizes. Or you could have several choices at a time, like with Black Market. Or the spells could each fulfil a common purpose, with only minor variations like Knights. Or perhaps closer to Ruins, make the spells cantrips, weaker, and gain two at a time.