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The price difference comes in because you're basically buying a new master cylinder.
the only difference between them is that there is an extra hole drilled and tapped, and a sensor installed. The cost of that extra manufacturing process is less than $10 plus the cost of the sensor, but when you buy one, you get the whole master cylinder.
the good news is, you can always sell the original "like new" master cylinder for around $150 and get most of that money back.