I noticed that woodcraft (woodcraft.com) is having a sale on 12"x12" baltic birch plywood the other day. the 3/4" and 1/2" thicknesses are only $1.99. I thought about buying some a few days back but didn’t want to pay the shipping. Well, they just sent me a code for free shipping.
I’m pretty sure the code was single use, but I’d bet that adding some stuff to your cart and waiting (you’d probably need to create a login so they know how to contact you) will get you the same deal.