Having a bit of experience here is what I know.
Sometimes amazon makes special offers. There can be 1 unit left or 5 of a specific product...
If it is 5 and somebody buys one, the price goes back up a bit. If you wait a few days more and nobody buys, you will see the price go down again. It is like this until one is left.
With one unit the price goes down until a certain limit (this is the gambling part: when to pull the trigger...) If nobody buys the last unit and the price is at minimum level, you will se the price go up for a couple of dollars each day for a day or two. This is generally an indication that the price will go back to normal soon.
Generally when you reach 40% off the regular market price you are very close to the lowest price...