I'd originally planned to buy the acrylic from a local source, as online dealers really ream you on the shipping. Unfortunately, the local source (A1 Acrylics) charges cutting fees now. They didn't the last time I was there, a few years ago. The only way to actually save money over buying online is to buy a full 48" x 96" sheet. Which is around $100, and way more than I need.
However, one thing they have added since my last visit was a scrap bin, where everything is sold for $1 a pound. Which was fantastic- there used to be a Cadillac Plastics in town, and they did the same thing. I used to use acrylic a lot, but when they closed, I no longer had a cheap source.
Black acrylic isn't as commonly bought, so they didn't have much in the scrap bin, but I was able to get enough to make two of the rooms and maybe a third to half of the corridors, for $8. If I'd wanted clear acrylic, I could easily have gotten enough to build every section for every mission in the game, for under $30. I'll probably go back in a few weeks to check for more. I'd go more often, but it's on the other side of town.
I have made one slight alteration to the design. My main reason for wanting 1/4" acrylic was that it was thick and stable for the bases, and the glue surface was sufficient for the walls to be pretty solidly attached. However, I realized that since I am now gluing the walls to the side/edges of the base pieces, the walls can be made from a thinner acrylic. I'll still get a good 1/4" glue surface and a durable base, since that's determined by the thickness of the base now and not the walls.
With any luck, I'll have some time this weekend to start cutting parts.