Emerald Eventing said:

I would start in the high end of tier 3. 30k+. I don't recommend starting with high prices right off the bat. Start lower and then work your way up as you improve and get more orders :)

I would work on grounding. In the first few pieces, border with the horse jumping out does look cool, but that's no substitute for plain old grounding. In the last examples, the horse looks like it's just floating rather than standing on the ground. If the horse is in grass, I like to use a soft eraser and a very small hard eraser.

Id also work on your lighting. You're off to a good start, but adjusting the lighting even more would make the horse look like it's actually on the background.

Good luck in your art journey :)