The best compromise for accuracy vs. cost is the Magnehelic gauge
by Dwyer.  They are probably about $50 to $75 new from Grainger, etc.
(they now have on-line sales) but you can usually find them new surplus
on eBay if you are patient.  I have purchased them for $15.  The cheap
Marshaltown gauges that used to be sold by PPCo are garbage.

For most player work you really do not need an accurate gauge as long
as it is repeatable.  You are most often just concerned with relative
measurements.  A water manometer can be made for very little money and
it is very accurate, but be sure to use a water trap jar to prevent the
possibility of sucking water into the piano.  I prefer a trap jar over
a float valve because it is 100% safe.

If, for any reason, anyone has a gauge that they want calibrated,
I have a Wallace and Tiernan precise gauge calibrator and can make
a chart of gauge reading versus actual pressure for any gauge.
I have no interest in repairing or calibrating the gauge itself; I'm
just offering this if anyone thinks they need it.

Wallace and Tiernan gauges can also be found surplus on eBay.  They
are monstrously expensive new but often quite reasonable on eBay.
I have a two-turn 10" diameter, 100 water column inches gauge that
I used to use for player work.  It is hand calibrated, with a mirror
scale, and is amazingly accurate.  Don't pay too much for one because
they can be damaged by overpressure or dropping, so a pretty-looking
gauge could be garbage.  Wallace and Tiernan are some of the best
gauges made in the world.

Best regards,
Spencer Chase
Garberville, California
http://www.spencerserolls.com/