As Notso says, effect of 1 missing is pretty small... In fact, the biggest "cost" of one missing is the inspection required every t/r to ensure the tape is still in place...
I think we have to add a small fuel penalty, even a small RTOW penalty, and I am sure that over many years, the marginal fuel saving pays for the winglet. However, whenever a catering truck knocks one off, we just carry on with a piece of tape in it's place <G> They are supposed to be replaced "at the earliest maintenance opportunity", but since we have no spares, they tend to fly like that for a few months...