I did some quick data analysis of the artist I follow. After going though more than 60 artist with listed commission prices, and focusing on a single character flat color (or whatever was more similar) the average price is $80 USD. Of course pricing schemes vary wildly as do style and skills, and it's really hard to compare the subjective value of a given piece. I didn't collect data on followers or time, but I could se the general trend that artists that have been longer on the fandom had higher prices. If I had time I'd love to do a deeper analysis, sadly a lot of artist have listing on private channels or behind subscription models.