Shep - While Live Sand would be the ideal, even at discount rates, sand for a DSB on a 100g would cost hundreds. I mean, even your friend paid $400 for sand.
The beauty of Southdown is it's price. Although higher on the West Coast, $15 per 50 lb. bag (which is what I paid last time) is much cheaper. Cost is a factor for me. Building the setup from the ground up, I am all ready into it about $1500 and don't have sand, rock or lights yet. It's filled with water, but that's all.
I got it before from an LFS who stumbled across a few bags, but I haven't seen it in a while.