I haven't tried this yet, but I'm thinking it should work. Make one of those FM transmitter circuits (usu. one or two transistors, some passive components, usu. titled FM Transmitter 1, 2, etc...). They work pretty good and can transmit 20 or more feet easily. Put a micro on it to transmit, at a very slow rate, a signal like you would see on a RS232 line: start bit, data, stop bit. Either have two frequencies, like a modem would, or just a frequency that equals a 0 and lack of a signal is a 1. On the recieving end, hack a $5 Wal-Mart radio (cheaper to buy the radio than build a reciever yourself) with a band pass filter (or two) into a micro, to recieve and interpret the incoming data.
I built one of those circuits I mentioned above, that had a mic on it, and I paralleled in a signal from a micro (square wave, though I also did it with the capacitor voltage on a 555, which was more sinusoidal though it made no difference). Even when the input from the mic was undiscernable, you could still make out the beep-beep-beeps from the micro.
The design should be very low power, if you put the micro in a sleep mode when it's not transmitting.
j.