Are you opposed to running silica based sand in your tank? I know back when I was up on things, it was looked down upon by the masses, but Im pretty sure some of the experts stated that it doesnt release silica at any rate that can be harmful. Im not up on the current status of that debate, but if your willing to take the risk, there are all kinds of sands available locally for the cheap
Also, I forgot the name brand, but WalMart carries a 'playsand' that comes in different colors. Thay had 'natural' that was a little brown for a reaf tank, but the 'white' actually didnt look fake. I think underwater, it could look great, especially if you mixed a little rubble in it.
Oh, and FWIW, you run some of the same risks with silica sand in FW tanks (silica fed algaes, etc..), but Ive never had a problem with it. Ive collected a ton of used sand from locals over the past couple of years, but if I need to supplement any at all, Ill be using my Lowes silica sand in the fuge......