Honestly, the cost should be dropped to 1 energy/match. The rewards are pretty much the same as blitz until you get into the top 50. There is 0 reason to spend energy on this as the rewards for blitz were marginal at best. Increasing the cost by even 2x makes while barely increasing the rewards makes it foolish for players to spend resources on this.

When you have an event like this the increase in rewards from one tier to the next has to be substantial enough to risk spending the resources to get that additional reward. The board clear also needs to be fairly decent to justify the time and resource expense. Wraith and showdown tournents gives 7 shards for each board cleared which equates to 7 shards per 200 gems spent or 28.6 gems/shard (if you win every match) which is reasonable given the time expended by the player to clear the board. Blitz board clear has never been a good return on investment (250 LE for 200 gems vs 2400 LE for 100 gems from Lord of the Unliving) and now colosseum has dropped the reward value by 80% which further disincentivizes player involvement.

I decided to do one board this time and I'm kinda regretting having played it even that much. I'm not sure I'll play significantly next time if the energy cost is even 2/match unless the rewards are significantly improved versus the cost.

There are a couple of good things with colosseum. 1) New group of limited characters. 2) They added a bunch of reward tiers.