Well the Leonardo w/ headers in the Arduino store is $24, but you can find one cheaper online at say eBay or DigiKey. Do is need to be a Leonardo? You can do the practically the same things with a Nano, Mini, Micro and Uno. All can be found online for cheaper prices. Make sure you pick the right ones, most if not all say Arduino compatible, but you can still run into problems.
The Arduino Playground has many libraries for various sensors, I'm sure you'll find what you need there.