I know this is an old thread but i'm looking for an easy to build inexpensive system for solving almost exactly the same problem. I've had quite a bit of success using the nRF24L01 tranceivers with the available RF24 library by maniacbug. I'm currently working on two solutions: one uses arduino nanos with the RF24 transceivers with leaf nodes acting as sensors and multiple hub/routers for receiving and aggregating data into my security system.
The hubs can overlap and even receive the same data depending on the distance away from the sensor but the system accounts for that. Another approach i've considered is to have each sensor act as node on a mesh network but that requires more work and the use of another library that i've yet to start looking at..
The benefit is that these radios cost from $1.50 to $3.00 apiece usually sold as a 2pc set for $6 where the Xbee modules listed above cost from $25 to $60 for just one.
I'm not sure if my response is timely enough but I believe its a better solution than the Xbee approach if cost is a consideration.