Mirror: ~110$ after shipping -Two Way Glass Mirror - 12" x 24" - Mirropane - 1/4" Thickness - Flat Edge Polished - for Smart Mirrors, Hidden Televisions, Security and Surveillance Applications https://www.amazon.com/dp/B01MSAZ3PN/ref=cm_sw_r_cp_apa_i_UAdiDbB7HTW20 Rpi 3: ~40$ - Element14 Raspberry Pi 3 B+... https://www.amazon.com/dp/B07BDR5PDW?ref=ppx_pop_mob_ap_share Monitor: ~ 70$ - Sceptre E205W-1600 20" 75Hz Ultra Thin LED Monitor HDMI VGA Build-in Speakers, Metallic Black (2018 version) https://www.amazon.com/dp/B00S8W8QMG/ref=cm_sw_r_cp_apa_i_.CdiDb9X0FX93 Assorted cables: ~10$ I spent about 20$ on wood for the frame. I got the code from here: https://github.com/HackerShackOfficial/Smart-Mirror
Total: ~250$
Rough build guide: Pull the code onto the pi with git. Follow the instructions in the README to get everything working. Disassemble the monitor. Be mindful of ESD and don't destroy the display panel. Set aside all the electronics; you'll need them, duh. Once it is free test it. BE SURE to test the panel at most steps to make sure it's not dead or damaged. Make the frame to the spec of the mirror. Add in support to make sure the panel will lie where you want it to in the mirror. Do a dry fit to place and route all the electronics. I used hot glue and masking tape to secure everything down and block out light leak. Lastly, go into a dark room and test it out. It'll show you where most of your light leak is.
I hope this is helpful for everyone. Sorry if it isn't comprehensive enough. Also, two things not included in the price were my priceless pals that helped with the assembly. Big thanks to LDC and Big Jo.