Synopsis
This article explains some of the reason that prices on products, options, and shipping may change after logging into the site.
Possible Causes
Causes include customer level discounts, customer level extended pricing, and exchange rates.
When logging in with a pre-existing customer, that customer may be a member of a customer level that receives a blanket discount on all products, or receives extended pricing. This will cause the price of the product to reflect the discount, extended pricing, or both depending on how the customer level is configured.
It is also possible that the customer is using a different currency than what is configured for your store. This is particularly true for sites that are using multiple currencies, or that previously used a different currency during setup. For example, if a site originally used USD as the currency, and then USD was disabled in favor of GBP, it is possible that existing customers may still be "stuck" using USD. In this case, even if USD is disabled, all prices will be modified by the USD exchange rate.
Solution
Ensure that your customer record is not mapped to any unintended customer levels, and that the discounts and extended pricing are correct. You can determine if it is a currency issue by using a different browser (such as firefox instead of IE) or clearing cookies and creating a new customer account.