Silver or Gold Chests for Au

statistically, bronze chests gives 7-10 Au with 16.45% probability, whereas one can win 70-100 Au with 3% prob. from silver chests, and 600-1500 Au with 14.3% prob. from gold chests.For each 1000 keys you spin on bronze, you may get 110-160 Au. To win 600-1500 Au, you have to spin,statistically speaking, at least 7 gold. That is 7000 keys, which may land you 770-1120 Au from opening bronze chests. Winning Au from opening silver chests is less likely. That said, these numbers were posted months ago from Pixonic (please see below the link below for the original post), and have recently been updated without the actual numbers being publicized.
warrobots.com/en/posts/157
Personally, I open bronze chests for Au. The reason is that I was very unlucky and got 0 Au from gold chests even after opening 10 back to back ages ago (although I won components for some useful premium gear). I would have made at least 1000 Au by opening bronze. Give both a try, it is statistics in the long run, and your luck in the short one afterall (you might get the jack pot in your first gold opening)