Keep an "average" price.  Every time a new price comes out average this price with the old price.  You will end up with a price that cuts fluctuations in half.  If you want to further reduce fluctuations make it a weighted average, with only 10% or even 1% coming from the latest price.

While writing this someone mentioned the geometric mean, I *love* the geometric mean.  I use it for situations where you are selling to a friend instead of a store.  It is a way for the savings to be proportional.