(A desire not helped by my sons mad luck at pulling the Obsidian Flames version from a pack.) My guess would be the Japanese version will start selling for around $300 pack fresh, and the international version next year will easily hit $200, before coming down to about half that
The floor is a 4-mana deal 5, and that's very playable -- but this will often cost two and give you some sacrifice synergy
Two cards that come to mind are the Rayquaza from EX Deoxys and the Umbreon from POP Series
Grand Abolisher is the most mana-efficient option, but Kutzil, Malamet Exemplar and Myrel, Shield of Argive provide creature-centric decks additional benefits for more mana
For most people, change, even positive change, is difficult to deal with
Here, Devour Flesh will trade one-for-one with the Invisible Stalker only because the Rancor will return to its owners hand