According to an investor on FiG, Psychonauts 2 needs at least 2 million copies sold in order for Double Fine to break even on development costs but only if the game is priced at $60. Same goes for investors trying to break even with the game.
When it was originally pitched to investors, the goal was around 1.7 million copies needed at $20 in order for investors to get almost all their money back and Double Fine to break even.
For some reference, Psychonauts 1 has sold around 1.7 million copies since its release although almost 800k was due to Humble Bundle sales alone.
http://www.neogaf.com/forum/showthread.php?t=1294935