Have you tried Craigslist for existing buildings? Sometimes you get lucky, though you may have to take it apart and reassemble it, plus transport it. Still, it may be worth a shot.
Another idea instead of building it yourself is to look at Home Depot, Lowe’s, maybe a local building supply and see what they have for the storage shed kits. You’ll still have to assemble it yourself, which takes time but isn’t that hard, or maybe they will assemble it for a price. You’ll have to do some modifications like put in lots of ventilation, probably a pop door, a roost, and maybe a window though your ventilation may make that unnecessary. None of that is all that hard to do once you get the concept. How you do that depends on what kind of building you get.
From your name I’ll guess you are in Texas but that’s a guess. To save some costs you could look at a metal building but you would really have to open it up for ventilation. Heat kills a lot more chickens than cold. A tremendously lot more. If you could put it in afternoon shade, that could really help. Metal is going to be a little harder to cut openings and secure them against predators but it can be done. Wood is easier to work with, at least for me.
To cut costs, you really don’t need a floor. You will need to have some type of foundation to keep the wind from blowing it away and you want it up off the ground a bit to prevent rot or excessive rust, but treated wood, cinder blocks, or concrete can take care of that. It still adds costs but you need to keep it off the ground to keep it from rotting whether it has a floor or not. Thinking about termites for wooded structures is probably a good thing, especially if you really are in Texas. Hopefully you don’t have Formosa termites where you are. Those are nasty.
Another option is to find a handyman that can build you one from scratch, foundation to completed coop. If you can give him a good description of exactly what you want, you should be able to get a lump sum price from him. Maybe paint it yourself to save costs.
For 25 chickens you are looking at a pretty big coop. It won’t be real cheap unless you get real lucky on Craigslist. Good luck!
