Is the one in the picture the solar one that she wanted you to convert to LED? Can I ask you... why do you think it's going to cost her so much? If the lanterns are built well enough, then why not just sneak a socket in them and add an LED bi pin, wedge base, or single contact lamp? Run your lead up to the hook and run wire along hanging staff to your 12 volt circuit.
TO buy lanterns like that are going to run a few bucks... and will most likely be line voltage.