I have some anecdotal evidence against selling "level packs": most people that I talk to hate them. There's a difference between buying things that enhance the game experience (powerups, character skins, etc) and paying to unlock the full game experience. When an app is free except for the first level, people feel deceived (unless you put "Lite" in the title or something).

Disclaimer: I have no studies or objective data to support this, this is based purely off of what I've heard from other iOS devs and regular humans.