Like look at Wrath... on one end it's a worse Darkbomb for 2 mana because it can't hit face. The other is a 2 mana Mortal Coil (a bit better than Coil but still priced higher).
With Wrath, you pay for the choice by not being able to hit face. With Starfall, you pay for the choice by not being able to hit face AND one more mana. If you made Starfall 4 mana, then its in line with how Wrath is costed.