Most places will charge per week or per month and the price tends to be from £10 to £100 per week. Location, amenities factor in but also how much you do. DIY (do it yourself) fields are towards the low end but you have to provide food, water and clean the poo up even in the dead of winter, do all or most of the work.
Full livery where they do everything provide feed, check horse, clean poo, water and everything else when you cannot get down can be as high as £125 per week sometimes a little more in pricy areas. Full livery often includes a stable and field. Most places are between the two.
EDIT: If its a large yard and you have a good house you can sometimes get a discount if you let the yard use the horse for public lessons. I don't like that but its an option. Another option is to loan the horse out to someone. Mostly its young girls but sometimes older people well want to loan a horse long term. They have access to the horse on set days normally 2 week days and 1 day in the weekend. Its still your horse but they look after it and ride it on those set days.
EDIT: Don't forget its not good to use the same small field all year around. Grass needs time to recover. I used to cycle my horse though 3 fields a year, my Mum on the other hand partitions her field.