To be clear: The $20 line I mentioned would be for off-the-shelf products. For a commission, I would be willing to pay more if the content warrants it (and it usually would - $20 doesn't buy a lot of work in the West, and I don't want to exploit people).
Again, it is unlikely that I am going to commission you because I need some very specific stuff that not every 2d artist will be willing and able to pull off (i can't go into details here but feel free to check out my game), but other people who are considering commissioning you will probably want to see a portfolio as well. That shows that you can deliver, and gives prospective customers an idea of what your style and skillset is.
Good luck!