dropdownlist issue

Question

hello;

how can i get charges of desire element of dropdownlist in my textbox .for e.g, if i selected sugery the charges of surgery display automatically in my textbox when i selected surgery. i am sending my code n scenarion image. one thing, i will tell you . i am not understanding i write this code in which place either behind the button or form load.

regards
sumaira manzoor