Ken makes an important point. Along this line, remember that the harmonica
produces a relatively weak fundamental frequency. The 2nd and 3rd harmonics
are very strong and influence the tuner quite a bit. That's another octave &
octave and a half higher respectively - so a wide range of sensitivity is
crucial for accuracy.
re: the original question
The Seiko ST-747 is a good tuner in the 20-30$ range and works very well for
harp - with great ballistics (doesn't jump all over the place) and I've
verified its' accuracy with the super fine Turbo Tuner.
The trouble is, in this price range, you're making a lot of compromises. In
the case of ST-747, it's strictly a "daylight" device - no backlight, and no
auto shut-off.
The convenience of auto shut-off can be a major money saver, if you're as
scatter-brained as I am.
Cheers, Staggerin' Jim Listen to Roots Harmonica at http://www.live365.com/stations/staggerinjim