Someone calculated that expected value is ~70% of buyin. That sucks. I did the math myself (I never trust math calculations on the internet if they are more complicated than adding few numbers
) and turns out it was more or less correct. The guy that did it assumed that golden legendary is worth 1600 dust. But I think you have to take into account your collection. If you already have all golden legendaries then one more is indeed worth only 1600 dust. But if you are going to keep it it's 3200. The bigger is your collection the less value you get from the brawl. To make it simple I made following assumptions:
buyin = -1000
pack = 100
1 gold = 1
1 dust = 1
golden legendary = 3200
win rate = 50%
Expected Value = -256
So if you are average* player you should expect to end with 256 less dust every time you enter Brawl compared to just buying packs.
* average among players participating in Brawl. We don't know what the average will be, maybe rank 5, maybe 2, maybe mid tier legend player.
That's expected value for all players combined. What about particular player with higher win rate?
51% = -208
52% = -156
53% = -99
54% = -35
55% = 34
60% = 511
65% = 1274
70% = 2438
You need to have 55% winrate to break even compared to spending gold on packs. On the other hand it's going to be insane value for top players. No wonder they capped the amount of times you can enter the Brawl
One more concern I have is that if you want to play in this brawl it basically forces you to convert gold for packs from WOG. That's 1k gold less for packs from next expansion. I would much prefer to get 100 gold instead of a WOG pack. So maybe this Brawl has even less value for players that don't intend to buy more packs until next expansion hits.