Later on in the game, RP production slows down considerably in relation to the cost of items. Maybe introducing a tower which boosts RP gain from kills within its radius would help this.
Another welcome addition would be a 'max' button for buying things. I stick to using x1 as choosing x5 inevitably requires me to go back to x1 as it is. A max button would save a few clicks after each rebirth.
Great fun game. I hope that you're still planning on improving it :)