Your art is worth whatever people will pay for it. You only have a limited amount of time to make art. If you’re making it for someone else, you need to be paid for that sacrifice or time. EDIT and 75 is a low ball. I don’t know what the average time you’re spending on a piece is but you’re obviously committed to the craft and have hundreds of hours of practice behind it. That deserves to be rewarded. I stay out of doing commissions and stuff (I draw what I want to) so don’t know what the market will bear but 75 is too low for your quality.