Forms Data Controls :: Calculate Inside Gridview Amount=price*quantity?

In my webpage i am trying to create a order by gridview here item selected by user from dropdown and on selction of dropdown price display in next to item dropdown label price.now user insert quantity from next grid textbox and next is label amount shold disply calculate amount on price and quantity.i also want to use javascript for this prob.what i did that item and price displaying and user make input from textbox now want to calculation but how textbox recieve input and amount label dispaly calculate amount.

[Code]....