I feed for our entire farm in the evenings, which includes my mare and 43 other miniature horses, donkeys, and horses. It takes me about an hour and a half to do that and refill waters (when needed) since about 20 of them are on grain and suppliments- and its over a large area. That earns me free pasture board for my mare, acess to the farm's cart and harness whenever I want, and acess to ride on Wednesdays and Saturdays. In a way I guess that would be considered 'horse sitting' since I check all of them for injury, feed, water, etc every day. I calculated how much I'd be earning if I were to be paid instead of bartering, and I think it equalled about $35 per day. Ofcourse, that's a huge amount of animals and I doubt you'd be doing that as a service, but just in case you wanted to know ;)
I've also farm-sat for people around my area a few times, and my rate is $30 per day for two visits, feeding, mucking/turnout , inspecting for injury, and watering. I generally am not asked to exercise the horses unless their owners will be gone for more than a week, but when I am, I usually ask $10 a head for a 45 minute session on the horse. I always schedual a day a few days in advance to go over feeding, scheduals, and work outs with the owner though, and make sure to have a 'tour' of where things are so that I don't mix anything up or have to bother them on vacation.