I have a book printed in 1994 (before digital cameras became commercially available), which explains the use of the expo disc as a cheaper alternative to buying a handheld lightmeter to convert the camera to an incident light meter as opposed to using it as a less accurate reflected light meter. I guess with the advant of digital photography, where white balance can be manupilated directly on the camera, it has been cleverly repackaged.
But at $300, I think getting a second hand minolta v flash meter would be a cheaper option since it will tell you the colour temperature at a glance.