Symptoms
During the checkout process, individual products show prices, but the subtotal, tax, shipping cost, and order total lines all display $0.00
Cause
This is caused by having the Localization.StoreCurrency AppConfig set to a currency which is not published and/or does not have an exchange rate specified.
Solution
Go to the Currencies page in the admin site, and ensure that the "Published" box is checked for your store's default currency, and that a non-0 exchange rate is specified.