After clicking on addtocart button the cost of product is not displaying

First Of All i want to tell you make your qustion more discreptive.
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Well in start i want to tell you we use database for this we create table in database which have many coloum like product_id,name,cost,quantity etc BUT REMEMBER prduct_id IS MUST AND PRIMARY KEY so we can send every where product id though querystring and can take easily value of that coloum (which product id query string have)
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as i know you cant find out the cost of product when its clicked on add to cart button or buy (AM i RIGHT?) I think YES;

lol you not clear that you using html content for this or server side components

my this answer will work if you use html content(button,textbox,etc);

EXAMPLE- i am showing Product_id,Cost,Quantity etc(everything in textbox);

as like ---- textbox1.value=product_id;
textbox2,text=cost;
textbox2,text=Quantity;
button1.value=add to cart;
now onclick event of button send product id or other thing which is primary key in your database AS QUERY STRING and then you can simply select product cost from database and print that value where you want