If you're publicly requesting that someone else make you art, I would highly recommend indicating forthrightly if you are or are not willing and able to pay for a commission. It is a commitment of time and energy for an artist to produce a piece of art, and if they are to be able to properly consider a request, they need to have an idea of what they will get in return for their costs. You may not be in a position to offer payment, and that's not a bad thing or wrong - certainly circumstances can prohibit that - but it's best to be upfront about that, and to acknowledge that it's likely to be quite limiting to potential takers.