walgreens often has discounts/coupons printed in their flyers. once you place an order you can then get discounts with that too. i also think some are cheaper if you place the order online. i have used coupons that are like 25 prints for X, ordered 30, paid the flat rate plus just a dollar or two for the extras.
i have heard that some sites like snapfish dont always give the best quality even though they are cheap and you end up reprinting.