One can find a variety of open source hardware for prototyping—Arduino, Raspberry Pi, Orange Pi, LinkIt, BeagleBone, and Tessel. Most of them are really cheap; you can buy an Orange Pi for just $33. The price usually depends on how powerful the board is and what interfaces it supports.
In addition to a board, it is possible to purchase a starter kit that usually costs about $70. It includes a few sensors, a small display, lamps, cables, and a breadboard for connecting it all without soldering. The price of a single sensor starts from $1.
