From what I have read, you spend the bank coins to open the tier. Then you have to make more bank coins to open another. Every day your guild should be making around 30,000 unless they change the payouts. In theory every few days you should have enough to open the mid tiers, etc.
Unfortunately, the way they have this; 30,000 is the maximum a guild can make during one day. That requires every person in the guild to do all the activities with at least one refresh and to log in daily.
If you have people go on vacation or miss days due to work or other issues, you won't make the 30,000/day.
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If you have people go on vacation or miss days due to work or other issues, you won't make the 30,000/day.