Check out Facebook Marketplace. I see a lot of coops there, occasionally you'll find something good. I picked up this one a year ago for $75. Just needed a good cleaning and tightening all the screws, and I added some extra bracing.
The only manufactured coops worth anything start at about $200. Even at that, they won't hold more than about 5 birds at best.
To get a decent coop for $75, you'll have to build it yourself.