If one can afford it, I see no problem.
70 euros? isn't this the replica shirt? there are 30 or so euros for the stadium shirt, which is of lower quality then the replica, but better then the fake shirts, and you support the club too.
Though it all comes down if you feel like supporting club financially. I like the idea that that I at least "chip in" to the club, but some may find that thats unecessary. It's just a choice.