Just because someone created something and released the sources to it does not require me nor anyone to update that creation for free or at a wage which is less then the value of a person's self worth.
It's no different than someone asking for money to script, alter the sources of the framework or even create sprites.
If I have to pay someone an exorbitant amount of money for artwork, sprites or other assets then if you would like to retain my services you will have to pay me on my terms. That is just how the world works.