Wireless Sensor Nodes for Arduino-based system?
The cheapest option would be to use a very simple transmitter like the one found here.

The problem is you'd still need to read the sensor somehow to be able to send the data through the transmitter but that could be accomplished with a microcontroller that is much cheaper than a full-fledged Arduino. Each sensor would use a wireless transmitter to talk to a base (Arduino-based) with a receiver that would then act as the web server. That might be a bit of a daunting task if you're just starting out.