It is of course your choice what to charge customers, if anything.
I often have the same thought when designing apps: "Is this worth paying even 99ยข for?" I then try to improve the app design to the point where it is worth paying for, if I would end up spending a lot of time developing it that is.
Have you considered taking it further and monetizing your hard work?
You deserve to be rewarded for the entertainment you're providing.