[blakecaldwell]

Satellite is also an option for remote locations:

https://www.sparkfun.com/products/13745

$250 for this module, and $12/month for the satellite usage.

For the sensor itself, that's a simple micro controller for under $10, and a $5 sensor, a battery pack, solar panel, and a most likely, a few resistors. Costs add up quickly if you don't want to run out and replace the batteries, and if you want it to phone home.