Bob,
At the low end of things are the thread gauges that are included in the vintage
Ace tap and die sets. If you are primarily working with common screws a good
used complete Ace tap and die set would provide a gauge AND the ability to
correct munged up threads or recut screws. I picked mine up for $20 US on the
‘Bay for a good complete set.

Just another option if you don’t need scientific or machinist accuracy. 

Brent

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Brent Kinsey