well, first you'd have to compare how much a sober person eats compared to a stoner with the munchies to establish a baseline of how being high affects appetite. Then you'd need to extrapolate that ratio with how much a horse eats sober to determine how much a high horse would eat. But more importantly, how do you get the horse high? I'd imagine you'd need something strong, like peyote or crack.how much does it cost to feed a high horse?
Some references for horse appetites:
http://theoatmeal.com/story/eat_horses