One of the cheapest ones I could find was Sam's (the wizard monkey) when she's next to ice king or flame princess. You need to buy fireball, then you can choose one of these. I like to pick ice.
I feel this is one of the best ones to choose as you don't have to go out of your way to get it, you will likely already have these characters and Sam's upgrades are very cheap, compared to Finn's or Jakes two person upgrades costing at least ¢1000 after other upgrades.