You wrote that "Magnification is an undefined concept at Prime focus."

It is not absolutely correct.
What is magnification in general - ratio size of the image to size of the object.
Name size of the image in prime focus y'. Size of the object is angular value and could be calculated as y'/f' where f' is focal length of the scope.
So we can say that magnification in the prime focus is y' / (y'/f') or just f'.

So we have a number to compare magnifications of different astrographs. It is a focal length!

Say my photo camera with a 50 mm lens has magnification 25 less then taking prime focus photo through Meade 10" (focal length = 1125 mm).