I switched from Debian to OS X in 2004, then from OS X to Ubuntu in 2014. The story of the switch back is told in full here
. That meant that the cheapest route for me was to install a distro on my MacBook, because I already owned it
:). And it really is good hardware, so I'm happy with the amount I paid for a high-quality laptop that fulfils my needs.
The interesting part of the switch for me is the question "why not use OS X"? There are all sorts of bugs in OS X and its applications, just as there are in Linux, GNU and their applications. The difference is that I'm allowed to fix the bugs in GNU, and other people can take advantage of those fixes. So I've been learning about GTK+ and Vala, as well as getting back up to speed with GNUstep, so that when I find a bug I can contribute a fix back.
Plenty of other posters have discussed that there are cheaper GNU/Linux-compatible laptop choices, and indeed had I not already owned a MacBook I probably would've considered some of those. But "cheapest" is a non-goal for me, or at least far down the chain below reliable wifi, good battery, solid construction and (to the extent that this is at all an option on any laptop) decent keys.