I took the time to sort out the total cost per sensor. These are all US parts from Adafruit or SparkFun except for the radios which I bought through the MySensors Ebay link. (It'd be interesting to compare the total cost with all China direct parts.)

Arduino Pro Mini 328 - 3.3V/8MHz $9.95
NRF24L01+ $0.94
Half Sized Permaproto Board $4.16
Battery Holder $1.95
Spring Terminal Blocks $0.99
3 AA NiMH $8.05
RHT03 Temp/Humidity $8.96
TSL2561 Luminosity $5.95

• Wire, Solder and two resistors.

Total: $40.95

I'm also excited to see the battery life. I'm hoping it will be good. Each sensor has a voltage divider on A3 thats measuring the voltage from the batteries.

@Anticimex I'm hoping these are as good of an option because I can configure them in different ways depending on what sensors I have on the node. I'm planning on building some with motion sensing, door/window open/close, uv, barometric pressure and 3d axis. The top part of the sensor would be identical.

@Damme Here is the same picture served statically rather than from OwnCloud:

I decided to use Adafruit's Permaproto boards as the platform for my sensors. They are $4 a piece or so and provide a lot of flexibility to attach sensors directly to the board. I've just finished my third sensor. Each has the arduino pro mini 3v3, radio, dht22 and luminosity sensor. They are power by three NiMH AA batteries. The Permaproto boards and the battery pack both slide into a 3d printed case.