Cheap arduino wi-fi TRIGGER I am looking for cheap way to have very basic wireless communication between an Arduino and a iphone.
I don't need to send complex data, it would be sufficient to "trigger" the arduino: let's say it is reading a certain pin voltage and waiting for it to be up in order to do stuff, my goal is to trigger this pin up by any (wireless and iphone friendly) mean.

I have figured out two solutions: wi-fi shield, cheap RF transceiver. The problem with the latter is that it is not trivial to communicate with a RF transceiver from an iphone. I should add another arduino controlling an RF transceiver on the iphone side and commanding this arduino with an usb.

The wi-fi shield on the other hand is too expensive. Even small routers such as the wr703n (30 euro more or less) is too expensive in my opinion. I just want to send a "bit" of information, so I don't want to spend more than 10 euro.

The third option would be to hack some cheap wi-fi controlled tool, such a lamp or plug: do you know any?
Do you have any other idea?

Thanks!
WiFi you either need a WiFi router or for either the iPhone or Arduino to create a network.

RF transceiver you need external circuitry.

The common way to do this at the moment is using Bluetooth Low Energy. Using a Bluetooth Low Energy (BLE) shield you can directly connect the Arduino to a modern iPhone or Android phone (supporting Bluetooth 4.0).

Check out redbearlabs, who have a range of shields and BLE-enabled Arduino clones. There are example sketches and iOS apps on the net. I have made a BLE controlled RGB light using the redbearlabs BlendMicro and example OSX application.