Well, the reward is huge (130T if you complete the challenge) and I agree it should be either hard or expensive to join, but they could keep it to 1T to rejoin. Every Token counts and it's painful to lose, but it's still not too much and you would earn it again the day after.

Some people would find it hard to win, and they will lose 12T every time.

I had to think hard myself to waste 12T to rejoin the first time I lost, even if I have a freakload of Tokens! I would rather use them to buy a player or green packs.