i nearly finished my mirror.
Wireless Stick 5€
Display Controller and powersupply from ebay ~30€
Display 1280x800 from old notebook 0€
Step Down Regulator from ebay 2€
PIR Sensor from ebay 2€
50cm HDMI cable 3€
some little stuff ~5€
I build this mirror on a CLAA154WB05AN display out of a Sony VAIO VGN-FZ31J notebook. Unfortunately it’s a glare display, which is not the brightest, but its working so far. For powering the raspberry i used a step down regulator, which is regulating the 12v input power from the display powersupply to 5v for the rasp.
For the controller switches i drilled some holes in to the frame and extended the switches with m4 plastic screws to reach out of the frame.