I'm currently building a coop similar, only I'm doing a gabled roof. The actual building size itself is 8x10, so maybe I can help you figure it out. Of course, I'm gonna use what I paid for materials here in NW GA, not sure how that would related to Nebraska pricing but still should be a general rule:
looks like T1-11 siding panels. for that size, 9 sheets. I bought the primed ones so they'll take less paint, paid $22.48 a piece for them.
simple 2x4 stud walls, probably done on 24" centers, without going back through the pics and counting, most likely in the neighborhood of $130 or so for them.
3 sheets of flooring, I used 3/4 OSB board T&G, $12.78 each sheet. I did my floor system on PT 2x6's, but could have "gotten by" with 2x4's. wasn't that much difference in the price when I figured 2x6's on 24" centers or 2x4's on 16" centers. 4x4 for the legs are 7.97 each, looks like you'd need 3.
I'm putting a metal roof on mine........when I went through my planning stages, I did a spread sheet for how many of each piece and a total including tax and my material list came up at $753. Now I did buy some cull 2x4's and saved about $80, and the metal dealer here in town has seconds I'm going to save some money on. I'm going to have right around $700 in the coop.
now the big thing is, if you build, your time. If you buy that one, you have to haul it. I'd lay it over on a trailer if you have access to one and haul it that way.
did they give any indication what they'd take for it? I'd start off offering them a few hundred dollars and let them know you'll be sure to clean up the area. They're probably totally indifferent about selling since it's a foreclosure, but make them an offer and see if they counter it.