Bummer...I want something light.
There are a few other options out there for this scale application. The pink insulation foam is pretty high density, is easy to shape (provided you have an electric knife, which is like $20), is easy to paint, and weighs a lot less than plaster.
Let me dig up some links for you one sec. (I'm in Iraq, so my internet is really, really slow. Like dial-up would be a boost.) While I'm digging through the internet, let me explain how I know about this stuff. I used to play a lot of tabletop miniatures games, like DnD and Warhammer 40k, real nerd stuff. And in order to make things more exciting, you need scenery. Well, you can just buy it, but as we all know that gets expensive, so you learn how to build lightweight, but strong and portable scenery.
So, the standard is, at least for tabletop games, is pink foam. The stuff is amazing. Plus, add in some Woodland Scenics flocking, trees and whatnot, you can create a pretty impressive looking battlefield for less than $50.
Ok, here come the links. I tired to get some of the good ones, but you should be able to get the jist of it.
Here is one on a wasteland. Most of that stuff was done with pink insulation and cheap styrene tubing.
This one is a nice example of what a little pink foam and some plastic parts can do for you.
This is a pretty good article on how to make modular scenery, so that you don't have the same layout twice.
This and
this should be some really good reading if they work. Looks like the company that taught me everything I know about scenery has decided to pull the plug on free knowledge. (Looks like the images have been pulled, but the info is there, old school style, just text. Yikes)
Anyways, if you have questions about it, let me know, and I will try to help you out.