We all know about Instructables, the social community hell bent on helping out others invent stuff, fix stuff and even replace expensive stuff with an inexpensive DIY version. TechBuilder has posted a video of Angelo, a 17-year-old physics major at DLSU, explaining how to build your own BB-8 with stuff already available in your home!
You control it with an Android phone (or tablet) and a generic Bluetooth control app that communicates with an Arduino. Total spending: 120$!
You can find all the details of the How-to at DIY Life-Size Phone Controlled BB8 Droid
And here is an extra, behind the scenes video.