The costs to maintain the stall and to increase/reduce capacity is determined by your stables racing class - the higher your class, the higher the cost to resize and maintain these facilities. Top class horses need top class care! No matter which class you are in, the first 4 stables will be provided free by the sponsors of your stable, with no ongoing cost to your Stable finances each week.
Some of the unused stalls in the stable (as determined at Financial updates on Sunday evening) will be leased to the public, earning you income. The amount of stalls you can lease to the public is determined by your stable's prestige level and the number of administrative assistants on your staff at the time. The better your prestige, the more demand there will be to agist with you. The more administrative assistants you have the better your staff will be able to cope with collecting payments for these services. (See Administrative staff)