Creating a simple cost threshold wont work because cards that are near the threshold and constantly going over and under would be nightmarish not just for us, but for players
Once players and collectors get a chance to start pulling cards, the market is sure to fluctuate greatly
Action cards like Fire the Cannons and Smash will help you keep Evasive cards off of the board, while three and four-cost cards with two lore like Jasmine, Disguised and Malificent, Sinister Visitor can keep the pressure on and force your opponent to trade instead of quest
Tannuk, Steadfast Second has a familiar Sneak Attack-like ability, not unlike Purphoros, Bronze-Blooded, and it should get people brewing around massive artifacts and red creatures
For cleaner source comparisons, use Pokmon card pricing sources before setting a final number