with the sensirion sht11, you have the temp AND the humidity sensor in the same package...
the sht11 is sold by farnell at 13 euros for 100+ qty
on the sensirion web site, they say you can contact them for higher qty. may be you can reach 10 euros for 1000+ qty ?
by the way, if you look at others humid sensors, you may get cheaper ones, but you will have a lot of components to build around to make it works. with the sht11, you need a small microcontroler only.