40k for those hours might be enough for a single guy who's out to make a buck doing something he likes.  If you throw in a truck, some gas, and a little beef you might even keep him around for quite a few years.  However, 40k won't be enough when his girlfriend or wife decides your farm isn't worth him not being home in the evenings and it damn sure won't be enough when he misses his kid's ball game for "your farm".  He'll price out the child support and/or alimony payments and just go get a job in town making the same hourly wage with benefits in an air conditioned chair for 40 hours a week without having to worry about his wife sneaking off to screw the neighbor while he's rolling up your hay at midnight before the storm gets there.

People, farmers in particular, greatly underestimate the value of getting home at 5:30, opening a beer, and forgetting about the rest of the world until you have to get back at it at 8 the next morning.  At the end of the day, you're buying someone's life and we only have one to live, so if you want to work someone like he owns the place, give him the light at the end of the tunnel or pay the man for his time.

You can't really compare your wages to his because at the end of the journey you're going to retire to a piece of dirt you can sell, or rent out, and he's going to retire to whatever he managed to shove in his back pocket after the bills are paid.  It might not make a lot of sense, but it's not uncommon for fast food franchise owners, restaurant owners, and small retail store owners to make less per location per year than the person they're paying to manage said location.