Forms Data Controls :: Running Total In Gridview Textbox C#?

in tat i was binding the table values like name,price and two textboxes for quantity and another one for total textbox for calulating total depends on quantity with price.

in my database table i have a row that contains minimum quantity=50 and maximum quantitiy=99 and price=0.66.and the min and max qty with price value varies.

i.e) if i enetring some quantites for example 51 during tat time it will fetch the between 50 to 99 price .ie)0.66 and caluclate the total value according to the quantity price i.e)51*0.66 and displayed in total textbox..

i dont want to use button to calculate the total value for each rows.

i entered the quantity at the same time the total will be calculated according to the quantity price.