Link to the Arena Pass FAQ: http://us.battle.net/wow/en/community/arena/arena-faq
It's $20 in North America and €15/£12 in Europe.
It's separate from the live realms, you can't transfer a character to or from the arena pass realms. You are able to create a level 90 character and gear it out with any gear they make available to you, including gems, enchants, and glyphs.
Only 3v3 counts for all rewards (including the pet).
You need to participate in (not win) 50 rated matches to get the Armored Murloc for your live account.