Someone else is doing the work. Cut rake and bale. I just provide the land. He will be taking the bales. So how much should I charge him for every bale he makes? And what would difference be if he spray all for weeds
Ok that's more clear!

It's hard to say as each area is different, you'd have to analyze the fields as to content, just grass & weeds or legumes & weeds or a mix and how many weeds are there and what others are selling comparable hay.
If you only provide the land, you're really not the the hay "Business" I think that you may be able to cover a portion of the land tax or maybe even the whole amount, but that's it.
View it more as a weed control than anything else. This is my view, but I'm not in your area so I cannot speak with knowledge.