With 2 arenas rewarding crystals, and regular rewards with Territory Battles and Territory Wars, the crystal income is steady.
...And you probably thought about buying fancy new characters straight with crystals. This infography gives you some elements to plan your next move.
For Free-to-Play gamers, buying a character from Datacards is unlikely. However, upgrading a character to 7 stars in the days following its release in the shards shop is now possible...
Shoutout to Warrior for sharing his own results, and to my anonymous guildmate who gave me his results a while ago.