You might be able to get away with 1/2" plywood, or thinner, depending on your joinery technique/technology. 3/4" gets used a lot because you can just end-screw into butt joints. For 1/2", you need to get fancier about how you build the box.
That said, go check it out and evaluate the hardware and level of finish. You can spend $80+ per pair on the drawer slides alone if they're the heavy-duty locking types. Add in the ever-rising price of wood and you could easily eclipse the "high cost" of that item by trying to make it just a bit lighter, etc. For reference, I just picked up two 4x8 sheets of 1/2" Baltic Birch for a recent project and my out-the-door total on wood and a small cut-charge was $140.
You might decide having something built and done is worth it. Sell the sub and offset your costs. Then, if you want to lighten it up later you can look at hogging out holes in the hidden panels or rebuilding the drawer boxes out of lighter material once you're feeling more confident in your abilities.
