I've only played today, at first disappointed that we no longer win 75 coins when we win. Then I noticed it's free play. I won two challenges and was reward 20 coins each. So the heading needs to be changed to "win 20" or upped to 60 for players that win.
Actually from what I see, they cost 40 coins to play and reward 60 coins. If you are not seeing a cost on the buttons could you please contact firstname.lastname@example.org with a screenshot?
Daily challenges aren't free unfortunately. If you look closely you'll see they cost 40 coins.