What breeds are "economical" feeders?

my horse has been at the same barn owned by an older man, who charges me dirt-cheap, since he was about 7 (he's 14 now). I'm moving, and my property has a fenced pasture and barn. Because I only have one horse, and my fiance isn't interested in riding, I was thinking about going to a local auction and buying a companion horse for my boy, or even a pony or donkey. What are some breeds that're cheaper to feed than the rest?