A little something I'm cooking up while I'm stuck inside on a snowy day. The building is color-laser printed on 110lb cardstock, scaled down about 75% from the full-size buildings available in PDF from Hawk Wargames' Dropzone Commander line. As you can see, very effective, and plenty sturdy, and I'd put them at slightly lower time-to-build than what I'd spend painting resin or metal buildings. Which are almost totally impossible to buy for 3mm scale anyway.
Overall, I think the effect is pretty striking. And by downscaling only half as much, you also can get them for 6mm scale pretty cleanly as well:
It's not what you'd get from a really well-made cast terrain city, but you can't top it for cost, speed, and ease of construction, and the visual effect in mass I expect to be pretty good.