Just my honest opinion here, I felt that this would not be a thing for long when I first heard of it and I only just joined. I manage an art group on deviantArt and have owned one in the past there as well, now with 8k+ members, I know that the artwork takes time and for that reason it should never be "free". If you want quality work and truly appreciate the work that the artists are putting in to do these services, give them a nice bonus on top of whatever they are charging you if it is cheap.
Artwork can be very expensive, I collect it so I know and I understand it. Try watching some artists stream their paintings and such, see how long it takes them. Calculate what they would earn at a minimum wage, more often than not you get your product for cheaper than what they should be getting.
That being said I wish I had more money than I did because I love artwork and I love to support those who love making it!