Oregon Scientific (among others) sells wireless hygrometers that are pretty good. The price is not bad -- they can be had for anywhere from $20-$40. The catch is that you need their base station (weather station) to receive the wireless signal. I had a weather station and it has 10 "slots" for devices (outdoor thermometer, wind speed, rain gauge, etc.) so I added some additional hygrometers for my humidors. They cannot be calibrated but after salt testing them they are pretty close.
It comes down to how much money you want to spend on all this. BTW, I still go into the humidors about the same number of times...