Welter seed sells 50lb bags of alfalfa seed for around $170 (depending on variety). I saw a website that said to plant alfalfa at 10lbs/acre.
I have watched a neighbor manage alfalfa for maximal yield. He likes to go in and spread phosphorous and potassium (maybe a little nitrogen) after each cutting. Will the landowner want to do this and who will be responsible for the cost? Since he's keeping the hay, I'd look to have him pay for maintenance (or cost share it). Otherwise, I think he's getting a great deal.
How many years will this seeding be managed? Alfalfa fields can get "tired" with age and need rotated out. Do you want to try to maintain it (financially) 3+ years? How long does the landowner want to hay it?
When will the last cutting of the season take place? If it is cut too late in the fall, you won't get much regrowth before winter. The farmer probably isn't going to try to cut too late, as that stresses the crowns and opens the door for winter kill. Still, there should probably be some talk on the date of last cut.