Black mangroves are only rare in the water
They are more of a semi-terrestrial species and don't like full-strength saltwater, so they're not as good as red mangroves for refugiums etc.
I'm not sure how much it works out to per pound, but you can buy frags of things like 'Red Caulerpa' (Botrycladia) online for like $10-$15.
I don't think I'd buy from that eBay vendor....
-R