-Food. There are always random condiments, etc, that are low or impossible to transport when they're open. You'll rebuy at your new place.
-If there is any overlapping rent, that's a huge cost
-As miss piggy says, there are always a *couple* things you need. A shelf to make a corner usable, that sort of deal. IME, this adds up to a couple hundred bucks pretty easily, even if you're pretty cautious about it. The smaller the space, the more specific the need, the more it will usually cost. On the other hand, the larger the space, the more of these areas to address. Can add up either way =)