a cheap way to build cool looking buildings is to find a place thats tossing out pallets (make sure they are wood worth nailing into) and haul them by renting a cube van for the day. Then just buy nails, put them together in the way that you like (make sure they will not fall, never make 2 story buildings this way). After that you might want to buy rolls of jute/burlap (or any material you may have access too) then stretch it over your frames. After that what we did was go to our local dump and they have an old paint bin, we jacked all the whites, and browns and yellowish colours. When u mix it it looks white usually because people use a lot more white then anything else, then literally drench the corners and ledges and main contact areas where people might rest their gun, hand etc. Then while wet just toss some dirt and sand on it. When it dries it looks a lot like adobe......Plywood is god, but it's expensive/harder to find
finishing touches make it look cool too, after all we go to all this trouble making our gear look real why play in some ugly crap.