There's no one bookstore which has the lowest prices for everything - sometimes a book is cheapest in one place, sometimes in another. If you're bothered about getting it at the absolutely lowest price you have to "shop around".
Personally I generally buy books at Fictionwise. They have an excellent selection, and generally good prices. They also have a "buyer's club" which gives you additional discount off their standard prices.
I note that "The Stand" is currently available with a 100% "Micropay" rebate at Fictionwise - ie it's effectively "free"; you get its entire price back to spend on other eBooks.