Actually, the Carib-Sea Aragamax is comparable in price (at least on the West Coast where SD is $15-$17/bag). It is like $18-$19 for a 30lb bag of small grain sand, and maybe a dollar more for the coarser 40lb bag.
I couldn't find Southdown this go around for myself either, and used the Carib-Sea. So far so good, and actually, it cleared up much faster than the Southdown.