The hardware isn't expensive (I run Linux on a phone that cost me 80 euros), the software development is.

Also, SailfishOS runs on some Xperia phones that cost around a hundred euros. The main issue with SailfishOS is that the OS isn't free software as parts are proprietary and they don't use a mainline kernel, so that you will probably bot be able to run an up to date kernel after a while.