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I strongly suspect a working setup could be made for <$50 in electronics parts.

The LabJack ADC, Phidgets DAC and Photodiode make up nearly $450 of the total cost. An Arduino, breadboard and a few components could probably do the same for a whole lot cheaper.

The concept of "low cost" here is relative to a dedicated lab watt balance machine, which is probably thousands of dollars.

Exactly, plus the actual stuff that makes the measurement.

The "shadow sensor and PID feedback" they're using could be scrapped and replaced with manual control. It wouldn't be as fancy, but it would save you the $60 photodiode.

The only other sensor being used is a current sensor, a.k.a a multimeter.

You could make a real ghetto version of this jig with a laser pointer, multimeter and variable power supply.

All the laser pointer does is point at the wall and show you the position of your balance beam.

Yea. A really enticing title. But disappointingly complex.