This one with 1um accuracy is about £63.00
You have to be very careful when reading a data sheet, it does not say '1um accuracy', it says 'switching point sensitivity' (which is what?).
I've attached the mechanical specs of the switch below. Its repeatability is shown as +- 0.001mm with hysteresis of <= 0.002mm. While these are still impressive sounding numbers the typical well adjusted machine might have backlash as low as of 0.01mm, add in less than perfectly clean activating surfaces, temperature changes in teh machine ,etc and the 'real world accuracy' of even a precise switch like this starts becoming less clear.
I'm not wanting to be a negitive nellie but rather want everyone to get the 'whole picture'. There are many factors involved and you can't just fixate on impressive sounding numbers from one sensor/switch and ignore the rest of the system.