Welcome to the world of arting - this is a really great first manip! I'm afraid I can't help on SB pricing as I have been a T1 artist for quite a while and am really out of the loop with starter pricing.
Here are some things I really like:
- the action/movement of mane and tail
- the tone of the horse (yellow/golden) matching the tone of the background
- the markings are a nice bright white
Here are some things I would work on:
- The horse
It seems like you accidentally chopped off some of the horse's booty on the right side of the stock image. When cutting, make sure to go very slow with a hard brush. It's important to stay true to the stock horse's form (so no cutting off pieces of body) and making sure your lines are flowing nicely. If you really zoom in on the portion you cut off, you'll notice that the edge is pretty bumpy.
- The hoof color
Hooves aren't golden. :) If a horse has white markings going down to their legs, the hoof is almost always a light coral/cream color. Studying other images of horses, seeing what tones/colors are actually present in horses in real life in their coats, hooves, and mane are incredibly helpful!
- Blend the white markings
You did the markings beautifully, but they look a little too crisp to be "actually" on a horse's coat. With a smudging brush on low strength, just gently work around the edges of the white markings, smudging them ever so softly so it looks like a natural line of coat hairs.
- Shadow & highlights on the horse
While you have a great shadow on the ground, a shadow also needs to be on your horse! It seems as though light is coming from the back in the background. So the "top" portions of your horse need to be highlighted in a lighter yellow/golden color. Likewise, the "bottom" portions of your horse, especially the belly and hooves need to be cast in a darker tone to shadow them. This will provide lots of depth to your horse.
- The mane and tail
It looks like you have tried to draw the mane and tail by drawing each hair individually. It is much easier if, for the base of your mane and tail - you create an outline and then color it in completely. This will prevent "white space gaps" in your mane and tail.
If you would like, please do visit my step-by-step art guide. I have all of these topics broken down and explained much clearer there, along with photos of my artwork so you can follow me through the steps visually as well. :)
If you have any questions, feel free to comment there and I can help you.