There will *never* come a time when you couldn't use a few more expansions to grow. We all do
.
They do get more expensive. First two are 200. Next two are 400. But then they are 600 each for the next 12 before jumping to 800. And so on.
See:
https://en.wiki.forgeofempires.com/index.php?title=Expansions
I usually buy them when I really need just one more to place something good, a new GB or event building maybe. Not just to make room for a couple more clockmakers.
What also works for me is to set a mental limit on how many diamonds I need to save before I buy the next one. And every time I buy one, I raise the limit. If I buy one for 200 when I have 670, then maybe I won't buy the next one until I have 10000. Then 1500, and so on. (I do the same with coins and supplies doing UBQs early on.) It's a mental trick that works for me. YMMV. Credit cards are always an option too, but not for the incurably frugal.