Recommendations for wireless alarm sensors?

I'm building an Arduino alarm system. I want to add some wireless sensors, both motion and alarm contact (door) sensors. There are lots of alarm hardware options, does anyone have any recommendations for a place to start? Does anyone have any suggestions for a wireless system that's easy to integrate with an Arduino?

I was really hoping to get wireless sensors for $10 or $15, and just have those transmit to some wireless receiver that can feed my Arduino. I was thinking something like this:


Other than cost, any reason you couldn't RYO (roll-your-own)?

For the comms part, you could use a single RF serial transceiver, and each sensor could have a seperate RF serial transceiver. You could poll each to send back a status (when checking for a status), otherwise, don't poll, and instead wait for a pin-change interrupt from the base transceiver. The sensors would only send when they detect a pin-change interrupt. Each would have an "Arduino" (best would be an ATTiny), with a pin connected to the sensing device (IR, magnet switch, etc); they would need to be coded in such a manner as to be in "sleep" mode and wake-up on the pin change interrupt (either from polling, or sensor status). Each would need its own "address", etc. This will likely cost a bit more than $10-15 dollars each (plus time and effort to build), though.