Drupal Commerce "let me know when the price goes down"
In product content type add a new radio field "Would you like to receive an email if the product price change?" with options Yes and No.

Install Token module and Rules module

You can write a rule to send an email whenever the product node price field is updated to only users who has selected Yes option. you can do this by adding a standard available condition or you can execute a custom php code and use Token to access the radio button value.