How much would this piece be worth?

Also need help with what I could do better :)

https://i.postimg.cc/26LRmcxf/Pon-y-woodds.png

Absolutely gorgeous

1. I would suggest blending the light on the tail more so there's not an obvious line.

2. Add more definition to the tail by taking a smaller brush and adding highlights/shadows (Make sure you have a low opacity brush)

3. I understand that Gimp and Photoshop have blur brushes if you happen to use one of those programs use the blur tool and blur out some of the background that's not surrounding your horse.

4. Add a layer to your image and create shadows/highlights on your horse and blend them out or turn down the opacity on your layer, your horse will fit into the background much better.

Not hating just trying to help. Hope my tips work for you! :)