Hi,
You have to retrieve the XML and the PHP files and rename them.
After that, you have to modify both files. In the xml file, you have to change this particular line:
<filename plugin="custom_price">custom_price.php</filename>
using the new name for the plugin and the new name (without removing the ".php" for the filename).
In the php file, you will have the chance the classname
class plgSystemCustom_price extends JPlugin{
and the line which load the parameters
$plugin = JPluginHelper::getPlugin('system', 'custom_price');
Finally, you can put the two files into a zip file and install the plugin in your joomla.
Unpublish the old plugin and publish the new one. That's it !
Regards,