Idea: Add 2 upgrades.
1 : Crit Chance: Starts at 500m coins (Increase Crit Chance +1% Per Upgrade) (Increase upgrade cost by this formula:

Value is the value of 1 block (discluding multis)
2: Crit Mult: Starts at 1M coins (Increases by 0.2x per upgrade) Cost multiplies by 1.1 Per Upgrade, Max is 10x Mult.
Hopefully you add this.