How can one justify spending $300 on a tuner just for the empeg?

The empeg tuner is not an FM modulator. It is an AM/FM radio tuner that allows you to listen to the radio through your empeg.

If you want only the empeg to be in your dash, for instance, if you don't have room in your dash for two stereos, then that's why you would want the tuner module.

Whether or not you want to spend $300.00 for that privilege is up to you. The tuner modules were originally much less than that, but they sold out of them. So the high ebay prices are due to demand exceeding supply, not due to any special advantages that the tuner has over any other car radio's tuner.

Keep in mind that Patrick will soon be offering tuner kits for less than that price. They will require assembly, though.