Built our own this year. Room size is 18' x 18'. Not sure of the cost, but I think it was around $1500. We put it in a corner of our barn, so that was two walls. I found 1.5" sheets of insulation cheap, a guy was stripping a roof off of a warehouse. Wood frame/siding was purchased locally. Finished the concrete ourselves. But a new/previously owned a/c on Craigslist, used a Cool Bot. I imagine the cost would have been around $5000 if you had to go and purchase all of the materials new, plus more if you can't do the work yourself.
This is what i did and it works great. We have an hay loft barn, one of those big barn, you know, any way we used the north leantoo of the barn and change it into a show barn. what we did was. ripped our the mantle that was in there than put dirt in there to level things out. next we strung a trap across the top for a roof. then i insulated it and put in a good window air conditioner and tin the inside. it is cheap and it works great. i have hair on my cattle like mad. you stll have to rinse but every one does. i have a couple of fans inthere to keep the air movment. this works and it is cheap and i still win a lot of shows.
My barn is probalby 40ft by 15ft and it stays about 70 degrees which is cool enough to grow hair trust me. let me know what you think.
I saw one the other day while looking at some cattle that was 12x15, I think those were the demesions. And the Guy cut a hole in the side of his barn and put a refrigerator unit off of a reefer trailer an dslapped it on there and insulated it and then put plywood up and he said when they used to show steers they would keep it at about 35 degrees and then with heifers they would keep it at 55 degrees.
If your using an a/c unit you dont want to use mister because they will work against each other. Make sure your building is well insulated!
We really need to come up with a good article on this subject - because it seems to come up quite a bit. If anyone has sometime to put one together; I can help proofread it and put it up on the front page.