There is a guy on the subguns.com forums that sells Whisper brass made from .223. He updates his thread often to keep it current. You should find it and if not just ask and he'll reply.
I believe it was about $100 per 1000 + shipping.
Here, I found it for you:
WTS 300 Whisper Brass 100.00 for 1000 shipped!!
Posted By: keith davis <keith__davis@yahoo.com> (68-185-130-56.midtn.chartertn.net)
Date: 8/31/04 23:40
I have once fired federal and remington .223 brass that has been reformed to 300 whisper, its trimed, sized, de-caped and polished to high luster, and no crimped primer pockets, this for my pre-post-pre sale I'm calling it, i sell this for 120.00 but to stock up some funds to get ready for the ban to go away I'm having a sale, so good time to stock up on 300 whisper brass for the gun you can put together after the ban is gone!!!!