I say: little.
How will they sell you DLC if you can just mod your game to squeeze more out of it?
Skyrim sold plenty of DLC.
Just make sure you release a creators kit that allows users to modify the official DLC you release.
iPhone = Great gaming device. Don't agree? Who cares, because you're wrong.
Final Fantasy VI (iOS), Final Fantasy: Record Keeper (iOS) & Dragon Quest V (iOS)