You are looking at £20-30 for a large rectangle bale and assuming horses are in the field during the day you would probably feed 5 horses for a week on 1 bale
We have big round bales of haylage which feeds 2 horses, 2 ponies and 5 or 6 cows for about a week. We make it ourselves but I think Dad would want about £25-30 a bale.
in South Oxon you can get fabulous haylage, not large squares but bloomin large rectangles for £20 each (buying 4 at a time). My local farmer has made some smashing stuff. A bale has been lasting me over a week with 4 horses and some out in fields during the day.