Oh wow, I even though about how you could've added a very simple difficulty system, where you would just start with a different amount of money, depending on the difficulty chosen, but seeing how you've added it so late, it makes sense to not work on such a feature. Still very awesome what you've got in the time you had!
Viewing post in Dragon Souls: the path of sacrifices jam comments
I also thought about a difficulty lvl based on the amount of money taken by the objects, the amount of enemies per wave and so. I wanted to have 3 different endings based on the amount of money left and make a real animation for the story. I made a system to create different waves in no more than 5 min, but sadly my obligations left me with no time to make more waves, and since it's only 20 waves I thought it wasn't much of a problem.