Getting a legendary is probably the single worst possible result.
It is on the same table as 500 glory, which can be used to purchase Spooky:
2x glory key
10x gold key
So right out the gate it is worth FAR less than other things on the same table. In addition to that, people that are completing these tasks are top end guilds and likely have at the very least 75% of the legends at Mythic already. So getting one is worth essentially 100 souls.
1M gold = 33k per person. This is one of three rewards, so it "cost" 11k (to be exact it should be more as tables 2 and 1 yield less, but I won't complicate the math). 11k gold would get roughly 36 gold keys, which is 20 commons, 7 rare, 7 minors, 2 majors. That works out to 170 souls + a few stones. In short, the "value" from getting this low chance reward is actually inferior to simply spending gold on chests.