hide the price for users that are not logged on?

Go to
magento\app\design\frontend\default\\template\catalog\product\price.phtml

 if(Mage::getSingleton('customer/session')->isLoggedIn()): 
//just above 
 if (!$_product->isGrouped()): 
//and close it at the end of the file 
 endif; /* if ($this->isCustomerLoggedIn()): */