For this piece in particular, I'd go at least 100k. I think people could get a better idea of your skill if you had a horse taking up more space and touching the ground/interacting with the background. Possibly with more color, too, because it's difficult to say how well you're doing in the hair department.

Much appreciated :) I have one which is on the ground, but it's currently for auctioning so I'm not sure if it's okay to post it here before the auction finishes.