having not read all the pages i will put this here in case it hasn't been said. in all likelyhood The Tasmanian giant freshwater crayfish (Astacopsis gouldi) is the largest freshwaterinvertebrate in the world. The species is only found in Tasmania is the rarest crayfish in the world.
unless you count troglodyte types which may live in only 1 cave system and have only a few hundred members.
oh also having looked back, rarer than that half and half lobster, are yellow lobsters. there afaik have only been 3 ever caught. their coloring is quite yellow. like a good mustard.
I think for the part of your world the rarest and expensive is all Cherax Sp from Papua. The exotic color and species of Cherax are awesome!
I found this french guy, please check his topic at http://www.aqualifestyle-france.com/t495...ish-krebse.
In Jakarta (Which is part of Indonesia) it's rather difficult now to find a good Cherax Sp now. Back in 2007 to 2009, Cherax "fever" was all around. We can easily found a good quality Cherax such as Blue moon, Hoa Creek, Manticola, etc. But now, I'm trying to contact a person in Surabaya who's still breeding all kind of Papua's Cherax.
I'll update the information later