You are unlikely to find a cheap hygrometer which is truly accurate, at best they give you a good indication of whether it's "Dry" "Humid" or "Really Humid". There is a good write up on RH by Dr. Stan Schultz, it's concerning the care and upkeep of tarantula enclosures, but the science is solid.
A good read for any cigar hobbyist
http://people.ucalgary.ca/~schultz/Humidity.html