I think that is a good idea. One problem is that I don't think that Paypal supports variable monthly payments.
What would be needed is something like a direct debit, where you authorise them to take between $3 and $13 dollars (assuming they post 10 comics in that month), and you let them decide how much they take.
Another option is that they post extra comics based on a donations bar. They could post 1 comic per week and extra ones everytime the comic hit the target.