So how are you going to know it's in a range? Do you have all the prices for the past few months memorized?
Let me give you an example. I like to buy coffee capsules for my coffee machine. I can buy them from quite a few different vendors. The price is typically 25c to 30c per capsule, I have seen them as high as 40c but not higher. Sometimes a vendor runs a promotion and they go as low as 15c but most of the time they are around 25c to 30c. I don't keep a chart of the prices I just know from memory that this is the typical price range.
Now. Suppose I want to buy some capsules, is it better for me to wait until I see them at 15c or snap them up at 40c. the mind boggles.