problem with datagridview

Question

window application
 in datagridview there is combobox which get data from database which show items...
when we select an item in combobox then its price will display corresponding price in textbox which is also in datagridview  and when we multiply the qty and price of selected item it will display price ....
all the controls for item,price,qty,amount are in datagridview...
and i also want to display the total amount of all items in the textbox which is outside the datagridview...
 
thanks