Just for the shed? Seems a bit steep that would be nearly £2500 a month before he’s bought any feed for themIts £2/head/day. Anything less and you're doing it for nothing.
Charge a head basis rather than the shed else they'll ram the shed full, use more straw and require more work to keep clean. Then the farmer will be moaning about how dirty they are.
Make sure they're decent payers else you're stuck with someone elses cattle.
If they start haggling then just go and buy you're own stock - you've already got the shed and straw and are happy to do the work. You might as well have the profit from it.