In a previous article, I have use an attiny85 + ds18b20 + rf433 to built a cheap temperature probe.
This time, I needed the humidity next to the temperature.
I then decided to use a dht11 (you can get it starting at 1.50€ on ebay).
Total cost is below 4€.
Here below the wiring.
Here some pictures. Notice that i used an old (but still good for my purpose) smartphone 3.7v battery.