It looks like the smallest of those WizNET W7500 boards is only $30[1], and includes a magjack on a carrier board (not shown in the mbed pictures) which is still fairly compact overall. The W7500 seems to include a Cortex-M0 that you can run your own code on, so it could be more interesting than the W5100 chips. I doubt it will end up being available in a $2 module, though.