The price will depend on a lot of variables.
Is there water/electricity connected?
What are the boundary fences like?
What quality is the grass?
Who repairs the boundaries when broken?
Does the field become waterlogged in winter?
Be careful, there are some very good, but also some very bad horse owners out there. I know someone who 'inherited' the horse on their field as the owner did a runner and just abandoned the horse in their field to starve to death. That is rare though!