The problem I see with every or nearly every prebuilt coop I've ever looked at is inadequate ventilation. It's a drag to spend $1,000 plus for a coop just to have to get out a reciprocating saw and cut vents in it; it's never as easy to retrofit something as it is to build it the way you want properly from the start. The rule of thumb I look for is 1 square foot of vent space per chicken.
And the other thing that bugs me about prebuilt coops is the manufacturer's suggestion about how many chickens their coop will hold. One of those listings above suggested an 8' by 6' coop could hold 40 chickens! Well, sure, but they don't mention you'd probably need to have them all debeaked. That's barely over 1 square foot per bird.
I've built two coops and runs myself. I just got done with my third and final coop (seriously!). I hired a contractor to build according to my specifications. I'm thrilled with the way it turned out. The builder had the right tools and experience to build the coop properly, and since this one is shed size, I didn't feel comfortable tackling something so big on my own.
To save some money, and also because I enjoy puttering around with things like this, I had the builder do the hard stuff (the foundation, framing, siding, roofing, etc.), and I'm handling the finish out myself (insulation, hardware cloth on windows and vents, painting, etc.). I did a price comparison between my custom coop and a comparable Tuff Shed in my area. The custom coop was about the same as the Tuff Shed even though the materials used in my coop were superior, and I know the workmanship is, too, because I've looked at those Tuff Sheds at the home improvement stores.
. If it were closer I'd be calling and setting up delivery!
