compare Product price value (before and after update)

Hi, all mentors, 

    I'd like to compare Product previous price after user update the product price, if the price change, I can use BP to notify related user . So, I add one column into product (UsrPreviousPrice) , and try to store the product previous price in it.

   However, I am new in object event handling and c#, but I believe it will help solve the issue, right now I don't know how to write the script task part. Please help clarify any errors.