Summary

Cheap HALL OMNIDIRECTIONAL magnetic endstop - Only TWO components, sub $2*

Instructions

I used the Honeywell SS451A - omnidirectional switch so you don't need to bother with magnet direction, hall switch (not ratiometric), Supply voltage 3 to 24V; only additional part is a 10k pull-up resistor (according to Honeywell installation instructions).
*Cost = US $1.92/piece or 10 for US $11.87 or US $1.18/piece. (DigiKey part number 480-3587-ND) anyway sub $2.50.

THIS IS IMPORTANT!!! Use a SMALL, TINY magnet. This will address the 'hysteresis.' Hall sensors typically have different grip and release parameters, for instance 135G Trip, -120G Release for mine; the smaller size the magnetic field, the less important this becomes. See the pics above to get an idea.

The sensor sinks current when tripped, so you need to invert the AXIS logic, I use Marlin with RAMPS, so it looks like this in line 233 of configuration.h: "const bool Z_ENDSTOPS_INVERTING = true;" for the Z stop, etc.

Sorry for the hack solder job, but the cost of a PCB would break the $2 mark. You can also add an LED if you want between the V+ and out pins with 250ohm limiting resistor.

Hallstop-O