I'll have to go through and calculate how much it finally cost, as I didn't buy everything for the project at once, and I bought items for other projects at the same time (including the 10" Compound Miter Saw that Home Depot had on sale for $68. w00t! That made the project much easier).

Off the top of my head, it included the following:

5 ea. 2X4X96" @ 2.62 ea.
4 ea. 12" X 24" glass @ 4.39 ea.
2 ea. 1"X10"X6' Pine Shelving @ 6.96 ea.
1 ea. 4'X8' 5.2 mm Lauan Plywood (lots left over) @ 9.99
2 ea. 2'X4' 1/8" Whiteboard @ 4.99 ea.
2 ea. Display Case locks @ 4.99 ea.

That alone is just under $75 before tax. I had screws left from previous projects and the 1/4" mesh Hardware Cloth, though for BP's in both you won't want hardware cloth on top (unless you like having BP's with abrasion wounds on their noses!).

The toughest part was getting the middle board lined up correctly to accomodate the sliding glass in both parts. I think if I were going to build another I would cut wood for two seperate "tracks" in the middle rather than using one hunk. I cut the tracks for the glass by passing the wood across the blade of the table saw with the blade set to make a shallow cut.

The basic design could be modified fairly easily to put more "stack units" in one set up, simply by cutting the main side uprights longer.