If your fellowship plays the spire for gold..you get 275 diamonds a week…plenty to buy yourself a lot of expansions
Small bit of math. Let's go!
The first 9 expansions sum to 14850 diamonds.
If they aren't, the cost just skyrockets from there on out to the tune of 300-500 diamonds more than the previous expansion. The 10th expansion is 3300 diamonds. The 43rd expansion is 18500 diamonds. The interval is 500 diamonds after expansion 18 @ 6000 diamonds.
In about a year (52 weeks) at 275 diamonds apiece, that's 14300 diamonds.
Let's go ahead and presume that because you're in a Gold Spire fellowship, that pushes you to reach the top of the Spire each week.
This is stretching it a bit, but I'm doing so to increase the benefit of the doubt.
Let's say that because of that push, you always reach the top and are eligible for the 125 diamonds you might get directly from the end boss. At a drop rate of 45% across 52 weeks, that averages out to (125 * 0.45 * 52) = 2925 diamonds.
Using similar logic, you can get an average of 5.2 Genies from that endboss. Let's round down to 5 because numbers. Those Genies at the top are rather lucrative: Each one across their 100 days of life gives you a 1% chance of 200 diamonds, a 2% chance at 100 diamonds, and a 5% chance at 50 diamonds. These odds are (as of the recent update) shown when you hover over the dice icon in the Genie's production panel. Let's round up to 200 total collections since you can collect once every 12 hours. Given this, you can average out to (200 * (200 * 0.01 + 100 * 0.02 + 50 * 0.05)) = 1300 diamonds. With five such moneymakers, that's 6500 diamonds in a year.
So that's a total of 23725 diamonds a year if you were topping spire AND only doing so if you weren't going to top it yourself if you weren't in a Gold Spire fellowship.
Now...
If you topped Spire
anyway without such a fellowship and said fellowship maintained silver, that's just 125 diamonds from weekly with the other benefits still being maintained. You just lose out on (150 * 52) = 7800 diamonds yearly, which reduces your earnings from 23725 to 15925. A fair chunk but isn't exactly devastating (or perhaps it
is devastating to someone else? To each their own, I suppose). This isn't even getting into the additional diamonds you could earn from MA or from some other smaller sources, which could possibly add up to quite a bit, but is far outside the scope of this wall of text, but is mentioned here because your overall performance in Spire could impact how much you get out of MA. Most likely. With some extreme edge cases being ignored by that statement. Edge cases which some of us would love to fall into (City made of Moonstone Libraries, and/or having a gazillion Red Panda Masters).
Anyhoo...
After the first 9 expansions (14850 diamonds), the next 5 expansions will cost you 16830 diamonds. Another year of saving? The next 3 expansions will cost you 15900 diamonds.
The full premium expansion cost chart can be found here, near the bottom. Do not confuse that with the City Expansions chart, which also lists an optional diamond purchase cost.
In conclusion: Yes, you can buy some. But "a lot"?
Press X to doubt
Unless you mean this in the most literal sense. In which a single expansion is, in fact, a lot