They're cheap because of so very few parts and you're replacing the built-in displays of expensive lens testers with needing a computer with sound card and a little elbow grease to record and interpret the audio signal yourself.
To build the test
1. Radio shack 276-1657 @ 2.99
2. cord from a low cost store $0.99
3. Free download $0.00..
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Total cost $3.98 plus tax and 10 min to put it together
They're cheap because of so very few parts and you're replacing the built-in displays of expensive lens testers with needing a computer with sound card and a little elbow grease to record and interpret the audio signal yourself.