I rent out some buildings of similar size to your barns, with lights and power, and secure yard/parking outside for £350 each/month.
As for your grassland, I'd be tempted to sell the grass for so much/ acre. If you look in the farming press you will get an idea of what people are charging.
If he wants your grass on a regular basis I would charge something like £100/acre/year. That way he can put as much fertiliser on as he wishes and then the yield of hay, and how many cuts he gets, is up to him.