In this case, is it possible that the windows might allow for just a
teaser to be installed in covering them and the rest of the wall left as a wall? Otherwise what about sewing together a bunch of 54" as I remember it sections of fabric in having one seam per say 52" running horizontally or veritically? That could than be as cheap as
muslin sewn together given the proper stitch.
Otherwise I at least think that a decent quality
drop of some sort, perhaps even
roll drop would be needed. Doesn't mean you have to invest in the most expensive threads per inch fabric either but such a large expanse of fabric would require contacting a scenery supply company for your options. A un-bleached
muslin drop might be cheap enough if you can go with something that's more natural in color than white, otherwise a un-bleached
drop is going to no doubt be cheaper than a heavy weight
cyc drop in being no doubt some form of scenic
duck fabric.
A few ideas to start with at least. Hopefully others will also post some.