Recently did a discount at my art shop and the slots were gone fast. Before no-one visited my shop but the prices weren't bad. 50k for horse avi. 60k for stable set. 40k club set. I feel like my art is worth that much at least but I don't know. I would like another opinion.

50-75k IMO. I think the only thing you should work on presently is blending the horse/hooves into the background.

Personally, I agree with your original prices. People like me are attracted to discounted art so slots obviously go faster xD but I find that as you do more art, people start to seek out your art and slots will fill faster. ;D