Usually on a DIY yard yo find some other bod that you can help out - if you look after theirs for a couple of weeks then there is no reason why they won't return the favour. If all else fails, book horsey in to a professional yard for schooling for a couple of weeks - that way you can go away without worrying and when you get back horsey will be going nicely too!
im in the process of sorting this one out myself for my first proper hol this year in december! i just like to think that when i care for other peoples horses / animals i like them to come back and shower me with praise about how great their pet looks and to know ive done my very very best - unfortunately i know this isnt always reciprocated!
If they are having to go to your stables without having any other business there, then I'd pay them somewhere around £15 per day or £100 for the week plus whatever their petrol costs would be. I wouldn't use just anyone though.