After selecting an item in DropDownList in Dynamic GridView

Question

I used DropdownList  in my Dynamic Gridview.

I want to auto fill Rate cell of Selected DropdownList Selected Item(Product name) 

 

As shown in ScreenSort , as i selected Eraser in Productname column , the rate of Eraser  gets auto fill on selecting the item in DropDownList using database table

 

Please Help