My mother is a hairdresser, and as such, in my life I've learned just how valuable tips can be in service professions. My mom makes most of her money each week through tips, often earning $100-200 in pure tips. At a tab of $65 for a hair dye job (which is what she does), a %15 percent tip is about $9.75; at about 2 hours of work each appointment, multiply that by between 10 and 20 appointments a week, it comes to between 100 and 200 dollars and between 20 and 40 hours of work.
Now, consider the artist in question. Say that he/she is very talented, professional and hardworking, and would likely put between 20 and 40 hours of work into a piece. The most expensive artist I've seen in my short travels within the fandom is Kacey Migami, and she is also the person who I think charges the closest to actual wage for her work. She gets about $200 for a basic piece, give or take.
Next, consider the talenter/hardworking/professional artist in question that does NOT charge normal wage for his/her work. Let's say the average price, on FA, about $50 for a fully colored piece that the artist may have spent almost 15-20 hours to complete. In this situation, I see it appropriate to tip an artist for their work.
I don't personally tip with every piece; I don't tip when it comes to badge, sketch, or ink commissions, and often I don't if I simply can't afford it.
But when I can, I try to go that extra mile to let the artist know that I appreciate the work they've done for me enough to give a little extra back.
What does everyone think?