One of my former Ph.D. students, Mallory Vestal, sought to answer that question in a paper that we just published in the Journal of Agricultural and Applied Economics. Mallory is a horse-lover, a former graduate assistant coach of the Oklahoma State University Equestrian team, and is now an assistant professor at West Texas A&M University. Here's the abstract of the paper:
Because "lower value" horses were those most likely to (eventually) head to the slaughter house, we anticipated that their prices would be most affected by the slaughter ban, and that's indeed what we found. Here's the impact of the ban on horses priced in the upper 20$, 40% . . and 80% of the price distribution.
There were a number of interesting side-results, like these . . .
Want to know my own view on eating horse meat? I hinted at it in this editorial.