There have been many lucky wild horses that have found their way into the lives of very loving families. There are a lot of wild horses still out there needing that chance to live a life free from fear. Unfortunately, some wild horses that do not find good homes are sold for slaughter. This is wrong.
In Utah, where I live, seagulls are protected, not because of their small numbers but because of their historical significance to the state. In the same way, we as a country should protect our wild horses, instead of turning them into a "cash crop."
It may not be "cost-effective" but cost-effectiveness is not always the goal and role of government. No doubt Yellowstone would fetch handsome prices at auction to private land developers.