That sounds OK depending on the barn maker and actual construction/material.
Water and electricity will add quite a little bit. Grading/site prep and permits can be a wild card too.
I’m in southern CA and have built a number of barns at our place. Depending on the materials and the configuration of the barn, $7-10K per stall, with everything, is in the ballpark for a good brand like MD or FCP with composite walls.
A simpler barn, like Castlebrook (drop in T & G walls), aluminum sheeting walls, plywood walls, etc, should be considerably less.
I have done well finding used barns on Craigslist and Facebook Marketplace, but I have an excellent barn guy to dismantle and reassemble. Used barns, good ones, often sell for about $2-3K per stall especially when they are on properties that are either selling and no longer have horsey neighbors, or a new buyer who wants it gone.
Many people have zero idea how expensive it is to construct horse facilities… especially when boarding is essentially a break-even business.