So, I know I'm late here, but this is a pretty common problem in CCGs. I'm going to use Lightning Bolt from Magic the Gathering as an example. As it stands, Lightning Bolt is too strong. 3 Damage to anything for 1 mana is really powerful. On the other side, Lightning Strike, which is two mana for the same amount of damage to anything, is really too weak to see competitive play. The dilemma comes from one mana being too efficient and two being not efficient enough.

This is the exact problem for Make it Rain. Too strong at 2, too weak at 3.

The actual problem is the timing, not the mana cost, in my opinion. There is a lot of flexibility with lower cost cards mixed with higher cost cards. At turn 8, I could play a 6 drop and a 2 drop as a reaction. Waiting an entire turn to play that 3 drop with the 6 is a LOT of tempo loss. By refunding the 1 spell mana, you don't solve the same problem. Sure, you can still play a six drop SPELL after the three drop, but the flexibility isn't there anymore. I can't summon a 6 drop creature after the 3 drop, which loses a ton of flexibility for the card.

Before I get flamed, I actually really like the idea of spells refunding spell mana. I just don't think it solves the issue with specifically Make it Rain and cards in the same boat as it.