I'd start with the Damage by Level tables (with Rules Updates!) on page 42 of the DMG (repeated on p185 in the "Creating Monsters" section).
My personal rule of thumb is that a vanilla power that deals damage to a single target with no other effects should deal expected damage such that it would kill a creature of the power's level in 4-5 hits.
This works pretty well in the heroic tier, but breaks down as you level up. Having powers that do 100s of HP-worth of damage at level 30 would make things pretty crazy.