I'm not boarding, but I'm still wondering, for future reference
how much should it cost?
what should it provide? Food and water, field and shelter, blanket and fly mask, bug spray. What else?
should you charge them for their horses to use your blankets, halter, lead, etc? In the cost of boarding for a couple more dollars?
How much it should cost depends on your location, your facilities, and what you offer. It could be $200, it could be $700. Basic board usually includes grain, hay, stall cleaning, and turn in/out. Some people charge extra for blankets and boots on/off, feeding supplements, holding for vet/farrier/whoever else or it is included in the boarding price. Some have a limit on blanket changes. Some charge extra for grooming, fly spray, lungeing, or exercise riding if the owner can't be there. I have not heard of the barn owner providing halters, leads, and blankets. Some barns require using a certain farrier or vet, others are fine with whoever you want, some schedule farrier visits for the whole barn while others leave it up to individual boarders. Some barns want all the horses to be on the same vaccination and worming schedule. Some require owners to provide grain. Some require boarders to portion supplements into daily doses. Some allow trailer parking for free or for a fee.
Consider how hands-on you want to be, how consistent you want care to be, what happens if boarders go on holiday. If your boarders are mostly working adults who can't do a lot of daily horse care or be there when vets and farriers come your service has to be different than boarders who are there daily and can/want to be very hands-on. Consider if you will reduce board in exchange for barn work. Think about whether or not you will take retirees, stallions, young horses, if you want a particular mix of geldings and mares, if you will offer individual or paired turn out or only group, if you will welcome outside trainers.
Talk to people about the kind of boarding they like and why and who your customers will be. If you have boarded, think about what you liked and what you didn't. Casual pleasure riders may have different requirements than performance riders, the culture will be different if your barn is mostly full of teenagers vs. working people vs. housewives. Some barns screen clients to create a particular atmosphere or to make sure boarders fit into the atmosphere whereas others accept anyone.