Has a lot of potential. The artstyle is cute and the overall idea is great.  However, there's some ironing out to be done;

Either increase the % rate or lower the cost of upgrades. The balance of it all is unrewarding. Maybe it gets better but almost unlocking my 3rd character and it's all still mediocre.

Maybe add an online feature like Room Ranking which provides gold/gem rewards based on your position.