If you are password protecting your galleries and a user logs in, a “Exit Gallery” link should display along with the Photos, Price List, Shopping Cart link.
Other 2 options to add a log out link is just adding “logout” at the end of the gallery url ex: http://domain.com/galley-name/logout.
If you are using the “Cart” page created by the plugin you can also use the cart url ex: http://domain.com/cart-url/logout.
Al links above assumes that you are using permalinks and you have an english language web site.