About 6.50$ from here. These are the same boards I sell on my site for 20$ but if you are going to be teaching a class you could have 8 of them for around 50$
http://www.banggood.com/NodeMcu-Lua-ESP-12E-WIFI-Development-Board-p-985891.html
They include all of the power circuitry for 5v and 3v and are easy to deploy in a project using a cell phone charger for the power.
Please post up if you find any problems with the docs. Looking to do a good scrub on them here soon breaking up to one page per command and having a nice list of the commands.
http://esp8266basic.com
A BASIC interpreter for your ESP
Working now on Autodrop3d. A 3d printer with automatic part ejection system. https://autodrop3d.com