Use power line to communicate between devices - simple communication
The best way is to use an off the shelf part. So you don't have to worry about burning your house down. That is why I mention X10. There is already a library to interface to X10

As how to do it discretely, look at the Mamba Shield it is a hacker alternative to the X10. You can see from their schematic, there is a lot to it. Note there are no relays. And they have taken HiPot in to consideration of there components and design. Note the Mamba does not have UL or equivalent. So while it is likely safe, it has not been proven to be. And it is just as expensive as the wifi shield.

The X10 is not cheap either. But it is proven. Not sure whats UL equivalent is.

More research show the X10 Pro products (different than X10) are actually UL tested. Where the PSC04 is equivalent to PL513 and PSC05 is equivalent to the TW523. both of which appear cheaper then X10's original product. 

Where as it is hard to get any solution much cheaper. If you do it your self discretely low volume pricing of part costs is expensive. And off the shelf items have overhead.

The cheapest way is to use a $4@ RF Link Transmitter and RF Link Receiver pair.