I think it is still expensive, manufacturers in China ship Android HDMI-usb dongles for samples at 55$ featuring 1.6GHz A9 dual core, quad core GPU, builtin 802.11n, BT 2.0 and HDMI output featuring Android 4.1 and more..
Good one and indeed, explains the basics to get someone started. I think what is missing though from most socket.io (and node.js) tutorials is code examples and full tutorials of -in browser- clients.
It really depends on the use case and the applications, some times it is an overkill to use a system like the RaspberyPi fo r just monitoring some sensors or controlling a relay switch. Especially when it comes to price, size and power consumption, microcontrollers are the only answer. System on a Chip devices can make great gateways for microcontrollers and supplement functionality by providing e.g., data processing, secure communication, etc.