There's no reason why you would have to charge extra per month. I don't even know of anyone that has personally been a part of that sort of arrangement. It sounds like having the horse in work and competing at a higher level is worth it to you, and that's perfectly acceptable!
If you're a big fan of this horse/rider combo, AND you're comfortable with where he will be and who will be caring for him, then go for it! I just personally wouldn't do it at such a distance. It'll be good if you're able to see the horse regularly, but I'd still strongly advise that you get regular photo updates just to keep your piece of mind!