The best part is they are all BOE! That means lots of gold for you, should you be lucky enough to bump into him. I've seen the mount go for about 50-90k, and depending on the epic you can get even 30k. The random profession recipe may also be valuable, but usually it's just a green jewelcrafting recipe worth no more than a few hundred max. Personally I've found Poseidus only once and the loot was worth about 90k for me.
His respawn time is currently unknown, and first spawn was seen no sooner than 72 hours after server maintenance, so relying entirely on this method to make gold may be a bad idea. It's wise to park an alt near it's spawn points and check them out from time to time.
Here's the recorded spawn points according to wowhead
And if you haven't been hunting rare monsters before, check out some of the other rare monster hunt methods. There's lots of useful info on how to make sure you won't miss a rare when you run into one.
Thanks for the lootshot, einsteiin!