Is that tons of haylage, or tons of Dry Matter? 15 ton of haylage(at 65% moisture) should be achievable. Much over 6 tons of Dry Matter per acre is hard to get, consistently, "here". Haylage should(IMO) be priced on a DM basis, if the dairy is doing all the harvesting you can,and should, charge significantly less per ton than those numbers NVDave was throwing out.

"Here", this past season, standing hay(buyer cuts, chops, and hauls) was going for $80-100/T of DM. That would be in the $35/T range for "as-is", 65% moisture haylage.

Fertilizing and spraying and spreading manure-all on your hay ground? Or doing this besides, for the dairy?

Methinks you need to take some of your new knowledge and have a discussion with the dairy owner/manager(both, unless they're the same person).