Under the Store menu, navigate to Settings > Security > General
Here you will see the option "Require Login For Pricing." Check this box and hit save.
By default the prices are replaced by a link to the customer login page.
NOTE: Prices will be hidden from all customers without a customer type. Customers will have to be "approved" by assigning them a customer type in the admin console after they create an account. Simply having a customer account (which anyone is free to do) does not allow them to view prices.