the lowest cost possible for a humidity sensor is a match with a wire at each end ...
you have to measure some samples, but it is very reliable
althought it is not very precise...
just put another big resistor in serie with it, and measure the voltage at the second resistor with the adc of the pic . so simple.
depends on your application...
out of that I like the sensirion sensors (shtxx)
they are not cheap (around 15 euros each) but are very precise, and very easy to interface, no adc needed it is all digital with a sort of i2c bus.