OK,
try to change your linq query to this,
var prices= from st in obj.Wishlist_tables
join X in obj.movie_details on st.movie_id equals X.Id
where st.username == Session["user"].ToString()
select X.price;
And then count it..
int count=0;
foreach(var objdata1 in prices)
{
count+=Convert.ToInt32(objdata1);
}
or
count = prices.Sum();
Hope it helps you..