As I heard it, "Want to make a little money in the horse business, start with a lot of money."
I sure this will ruffle someone's feathers but here I go. There are basically 2 disciplines in horses - Western and English. Western people treat their horses as livestock. They get worked, are allowed to dry off and occasionally get hosed off. They get fed and turned out to pasture, with some hay occasionally. Western horses are waterproof, which means they can stand out in the rain and walk in mud. The can be hauled in a stock trailer with the saddle on. Most Western folks are reasonable to deal with.
English is a whole different breed of horses and people. The warmbloods have been bred for dressage, hunter/jumper and eventing. If those words have no meaning to you do not get into the horse business. The horses are treated like pets and no poodle every had it so good. They are fed 2-3 times a day, must stay in a clean stall at night with a fan when it is hot and have a blanket on when it is cold. The owners are usually a husband/dad that bought 1 or more horses for his wife/daughter. They are then elevated to the status of DQ - Dressage Queen. The DQ will never be happy with the way the horses are kept. And you know what that means, if momma ain't happy, nobody is happy. They show up, ride, then must give the horse a bath in an enclosed covered wash rack with hot and cold water and a fan to dry the horse off afterwards. They leave stuff everywhere for you to clean up. You must provide a secure tack room with saddle and bridle racks, at least 1 per horse. An English saddle starts at $1000 and goes up to $10,000, so theft is a concern. English horses are not waterproof, so if it is raining or muddy they have to stay indoors, so you have to clean stalls twice a day, so you have to dispose of a trailer load of muck every day. Stalls must have shavings or pellets over floor mats. You must have a fly spray system as 1 fly in the barn will cause complaints. The trailer must be enclosed with tack room and the trailer stays at the barn so have a place to park everyone's trailer. You must also have a clean bathroom with AC for when nature calls. And so on and so forth.
Most barns provide board with grain and hay included. Worming is a pretty standard requirement. That way you don't end up with a horse with no feed because the owner "forgot to get feed" and a horse that colics all the time. A warmblood will eat at least 1/2 a bale of hay a day. If your 40 horse barn is full that's 20 bales a day. You can get horse hay right now in East Texas for $8 a bale. Last year it was $10 if you could find it. So hay alone is $160-200 a day, plus grain at $15-20 dollars a bag, shavings at $5 a bag, etc, etc. If another drought hits and hay goes up you eat it, as you cannot raise board rates every month.
I would investigate it very well before getting into the board business. Horse are 24/365 business. There is no day off in the board business. For most it is a hobby that creates income. They have horses so taking a few more on is no big deal. Go volunteer at a stables near you and you will quickly get an idea of the horse business.
Got to go. It's 7 AM, the wife has left for work and I have to go feed, pick feet, fly spray everyone, put on fly masks, turn everybody out to paddocks, and clean stalls and the stables. There is a chance of rain today so I have to stick around as our horses are not waterproof. I have to hang the new sign I got, "I work hard at my job so my horses can have a better life."