lovetofrost: I just actually made a square one for a wedding I did this morning.
I called a local glass company, and asked them what the cost of some 1/4" plexiglass would run me in several different sizes. The glass totaled about 55 bucks, including the finishing of the edges so they weren't sharp.
I then went to the hardware store and bought a piece of threaded rod, the kind you would use if you were building a lamp, drilled small holes the size of the rod in the center of the plexiglass, used some 2" PVC pipe that I had used an ivory spray paint on cut it into 4" pcs. and used that as the separators between tiers.
I would say all total it cost me about 70-75 bucks, and I didn't have to pay any kind of shipping. The thing that took the longest was waiting for the spray paint to dry. I'll post a pic when I get it back from my client.