A less expensive, less messy, and possibly more durable option for what you're doing is to use masonite and cardstock paper.

Set your building onto the masonite and trace around the walls, inside and out, with a pencil. Use the outside trace to cut out your piece of masonite (or use a larger piece if you want some exterior stuff modeled against the outside of the building's walls, like a weapon rack or rubble or something). The inside trace is used to determine the boundary of where you'll need to apply a floor texture.

There are many methods you could use to make the floor texture. Cardstock is the cheapest since it's pretty much free if you eat anything that comes in a box, but you'll have to cut the tiles yourself and glue them down (though this can produce interesting results, especially if you make some of the tiles cracked or broken by chopping into them).

You can use household this-and-that to make other floor effects. Want a floor vent? Cut a small bit of window screen material (ideally not from your actual window, if you've a wife she'll clobber you over that idea) in a square shape and frame it in a rectangle of cardstock. Just add a little bolt to each corner (you could drill a tiny hole to represent an inset bolt, or glue on a tiny bit of plastic rod to represent a protruding bolt) and glue that down onto your floor in place of a tile area. A few toothpicks glued side-by-side and then cut into a square, then topped with a hollow square of cardstock, can represent exposed conduits.

There's lots you can do with imagination and household stuff that can get the job done. So if plaster and molds isn't going to be a consistent medium you're going to use, consider other alternatives first. This isn't to say that plaster and molds are bad, of course - they're great, just not the only solution and one which comes with an investment in materials and molds.