Adding new feature for BlockManager

I want to use grapes.js to create a website constructor for clients and when they show the design it should show the price of constructed website according to how many elements are used. What I am struggling in is that I can not create variable called "Price" and when from Blockmanager something is dragged and dropped that variable "Price" should change. Which line of code should I change to add "price" variable for every separate objects of Blockmanager and when object is dropped to specified place variable "price" changed? Thanks in advance ! Hope that this kind of feature will help others...