Building your own Smartphone? That could be a great hobby project. One way to do it could be to use a Raspberry Pi and use the Sailfish OS. In this video, Aleksi Suomalainen shows you what he has done.
I downloaded the Sailfish image from the Sailfish on Raspberry Pi2 blog. It worked for me. But not as good as I wanted it to. It is still a bit immature.
My intention was not to build a Smartphone anyway. I just wanted to see if I could use the Sailfish OS on the Raspberry Pi for something else.
But building your own Smartphone is a very cool project that could be done for less than $ 100. Not that you want to carry around the brick that you have built or that it would save you any money. It is just a cool thing that you can build.