Sheep grazing in Hampshire/Wilts for large parcels is 20p/head/week. The better the land, the more it will cost.
Converseley, on a plot that size, if it wasn't close to me, I would come to some agreement where no money changed hands - ie throw them a lamb, do the odd bit of fencing.
If its suitable for horses, however they will expect a lot of cash, until the horses trash it, of course and they need some nice sheep person to come and tidy the place up again.