I feel like I could have gone on indefinitely if I'd concentrated on farming bats rather than trying to push further and further to the right, but maybe not. The number of bats which spawn per room seems to be random, so it's really a matter of how many trips it takes to get enough coins for the next level of healing.
There is the basis for a strong, fun game loop. It would just need more enemy types and deeper combat, I think. For a jam game this is about perfect.