These are used in mass quanties by crafters that are leveling goldsmithing.
Mythril Ingot - 4 x Mythril Beastcoin
Keep the price of the ingots in mind when trying to sell these. The crafters that are buying these dont want to lose gill while lvling this expencive craft. So look at the prices of the ingots. A stack of M.beastcoin should be 1/4 or less that price (since it takes four per synth).
If your going to try to charge more then that, then you must expect the price of the M.Ingots to go up as well.
Very high steal rate on mid-tier Sahagin in Sea Serpent Grotto. Can also farm mobs for the Mermaid's Ring quest (see http://wiki.ffxiclopedia.org/wiki/Mermaid%27s_Ring) and Shall Shells whilst you are there, plus Treasure Chests and the odd NM.
They have a VERY VERY high steal rate off the Robbers in Altep near the OP only take me about an hr to get a stack with no mis-steals. So exp and gil at 45!? Cant beat it.
As of Oct 5, 2011 stacks went up to 30k on Quetz, yet a single still sells for 1k... is it really so hard to pick up a calculator and see that singles should be at least 2k, if not 2.5k... this server, is retarded.. yet again.
I don't understand why people sell these in stacks! lol, a stack is only worth 20k (maybe less) and if you're selling the singles at 3k a pop um,...hello! That's 36k if you were to sell them in singles. Doesn't take that long to sell them either. Be smart!