Well, this is my Idea..
For the clan upgrades,
Name Cost Upgrade
+$5,000,000 Max Vault $500,000
+1,000 Coins Max Vault 75 Coins
+100,000 Points Max Vault 10,000 points
+10 Max Members
This price increases everytime
you purchase this upgrade. 520 coins + 10,000 points

There's something like above.
My suggestion is..
Make The Quantity,
This mean, we don't need upgrade it one by one..
Same like Clan Rewards, there's quantity there..
So, Why not make it with Clan Upgrades :)

For example :
You want upgrades your max cash in clan, from 5M to 200M
It'll take really long time to upgrades it, but if we can use quantity, it'll take few second :D

So, what you guys think ?
Comments please :)

I think that this suggestion have been suggested before.