lineCurrent = (Voltage of the current-sense resistor) / (resistance of the current-sense resistor)
Considering a current sense resistor is fairly cheap (http://octopart.com/13fr200e-ohmite-988758) it shouldn't be terribly expensive to do. You should be able to throw that between a Vin- and Vin+ on a linear optoisolator and send the values back to a microcontroller to be reported.
I'm not an EE though, so grain of salt.
There are a few more components for building a basic board: https://instruct1.cit.cornell.edu/courses/ee476/FinalProject... http://enerjar.net/Hardware.html
Building a more accurate power meter is pretty involved: http://coolarduino.wordpress.com/2013/02/08/power-meter/ http://coolarduino.wordpress.com/2013/02/27/power-energy-met...
Both have online load/consumption monitoring (I wonder how exactly the Zuli stores/handles that when there's no app around?), which are probably the biggest thing separating them from existing remote power outlet products.
(For logging power, the only relatively cheap options I've found is hacking a Kill-A-Watt sensor chip (see: http://www.mikesmicromania.com/2013/04/moteino-kill-watt-har... ) or buying a Watts up? PRO - both are more much more expensive/involved solutions than either of these two new devices.)