I don't know if the hardware stores have it out there, but my local Ace Hardware has a cart out in front of their store everyday with mis-mixed/mistake gallon- and quart-size paint for $6 and $2, respectively. All sorts of brands in varying colors, but my favorite indoor paint is the Benjamin Moore Aura line, which normally retails for $50/gallon! If you don't care about color, it might be worth asking if they have something like this in the back/storage area of the store if you don't see it in plain sight.

Mitsy --- We always look for the mistake paints, but it seems like lately there never is any! I guess people have been picking up what they have mixed.