Help me! Need a simple low power, low cost wireless "On/Off" switch!

[DivineChaos]

Hey guys,

I recently posted a thread over in the Beginners section about how to design a simple wireless on/off switch. I have had some suggestions, but nothing really worth merit, so I've decided to come over and ask the wireless experts 

I think the biggest hurdle is perhaps I may not be explaining myself clear enough.

I have two "slave" devices A.K.A LED's that need to "know" when a third device, a USB dongle, has received power and when the USB dongle has lost power. Basically it is as simple as that. But it doesn't seem to be that simple

-- USB dongle is powered, sends signal stating that its powered, and when the USB dongle has lost power, the receivers decide that it is then time to turn off because they are no longer "hearing" a specific signal.

-- By "know" I mean, to receive the signal and be able to "tell" an MCU (or whatever is most simple) onboard the slave device to turn ON an LED, and when the USB has lost power, to "decide" to power down into "listening mode" and turn OFF the LED.

Some google searches reveal that there are MCU transmitters. Do they Receive as well as transmit? This may not work because the "brain" cannot be inside the USB dongle because when it loses power, well, it has lost power and cannot do anything. It will not have battery backup.

Other searches reveal that there are MCU transceivers. This would work for the slave devices, but they are expensive ($5 each?) and overkill. The slave does not need to transmit anything, only listen, and I'd need 3 of them and that would drive the cost of my project UP by about $15.

A perfect match would be an MCU Receiver. It would eliminate needing a receiver AND an MCU separately because it is integrated, but alas, I cannot find them.

Is using an MCU even necessary for just turning an LED on wirelessly?

A couple things of note:

1. No line of sight.
2. Minimum of 30ft signal distance through a wall or two made of wood and drywall. Maximum distance 50ft. This is negotiable.
3. Relatively low cost. My total budget for the 3 devices to function is around $10.
4. Receivers A.K.A the LED's can tolerate an additional load of 3.3v @ 20mA
5. USB Transmitter can use a maximum of 5v @ 500mA
6. Legal use of "wireless technology" because this MAY someday become something that I could sell.
7. Hand solderable with a heat gun and/or Hakko 888d, for prototyping. I *think* I can solder TQFN well enough. SOT-23, .5mm pitch is easy for me.
8. The most important of all, KISS. (keep it simple stupid)
9. Size limitation. USB dongle can't be any larger than a standard thumb drive (basically the size of your thumb) and the receivers cannot be larger than your two forefingers side by side in length and height. (basically about 1/2inch taller than a standard 2x AA battery holder and about the same width)

Can you guys over here in RF land help me out?