When we approach you for an art job you've put up, we do not want the "opportunity to shape your free RPG game" or "design the characters however we want" or "have input into the workings of the game" in exchange for a small budget. Maybe that is a good opportunity for a hobby artist who you can't 100% count on to stick around, but for an artist used to doing work for money?
That is called work. Work that you are calling a "favor to us". Adding design work to a project is not a favor. It will in fact take us more time to think of what to put in your game and THEN draw it than to just get a list of art from you. Some people would like to draw what they want for other people, but most artists already draw what they want! Our ideas are for our own games and they took a long time to think of and design in a coherent way. We don't want to "just draw anything" when we are taking on paid work. We charge a design fee when people make us do the design work because designs take time. If you would like to pay the design fee, we will gladly design things for you, but for a lot of artists? The design takes more time than the execution. Our sketch artist can pump something out in 20 minutes with a good description. Any part that she has to think about takes longer than that.
A small budget? We can work with. But don't offer us a chance to design your game for you (read: more work) in exchange for less money.
Instead, if you know you have a small budget and want to entice your artist to continue working for you, have a clear cut design for your game already worked out and a list of assets you will need. Be willing to compromise on how some pieces will be done, and don't ask the artist for more than 2 miniscule edits without expecting to pay extra. Allow the artist to retain most of the ownership of all their work. Those are things that artists value; things that will lower the time they have to take on your project, so they can still make a decent hourly even on a small budget. Adding to the work they're doing and then also paying less? Not so much.