Near impossible to say how much. So much depends on name, condition and style of free bass... You have the converter system which turns a 120 bass into a free bass or an all out free bass which is 185 bass with 3 dedicated rows to the free bass.
Now, I own a Hohner Morino VI N 185 button free bass and have some obvious preferences (predjudices... lol).
How hard is it to learn? Well if you are a piano accordion player, a small way to look at it would be like starting over completely from scratch on the right hand except it is on the left. The right hand stays unchanged and the left plays 3 rows of a chromatic button accordion (in a manner of speaking) using my system. I am not familiar with the layout of the converter system, never tried it. In short, it's a helluva challenge, but you can literally play piano music and even do things that a piano cannot, like play the same notes on the left and right hand (there is an overlap of range). If you play a CBA, it is a touch easier, but still not a walk in the park, but I believe that anyone can learn anything, if they want to
I think it is a wonderful system, but use it a lot less now than when I was in my conservatory days. I hope to eventually touch on it more in the future. In terms of popularity... I would rate it in the very rare class compared to stradella. Not many out there, but the few that are you will see they are mostly classical music stylists.