package per extension , yes , you can have control of price of every extension .
but can you set a different price for renew , transfer , registration ? you can't .
can you offer free domain (extention) + hosting ? and the same extention is paid when it is alone (no hosting)
nowadays a lot of new extension in the market , so for me a package per extension can lead to more than 100 package .... so that is a lot for maintain the package under control .