I've noticed this too. What caused my income to dry up was completing terraforming on a series of worlds without opening up new avenues of colonization to drop more infrastructure. The question becomes: should one ever terraform beyond colony cost 2?! If it is a financial disadvantage to do so? Interesting concept. Perhaps civilian trade goods shipping needs a boost for terraformed worlds!
There are two major reasons for my crisis: I geared my civilization for astonishingly fast industrial development, and ZERO civilian mining centers. Those are actually important wealth makers early game-- got my first one just last night-- i must be 25 years in or so.
As for financial centers, the world I talked about that I converted to a finance world had zero function. No minerals, only suitable for colonization. The other two worlds in the system are a massive mining installation (standard mines, 1500 of them) and a people farm that just exports citizens with a huge +pop bonus on the governor. The problem with my depression (which is ongoing, incidentally) is that my massive investment in construction factories at the homeworld far outstripped the ability to tax... but I want that production to get the mineral output really high (i'm burning through about 100k corundium per year at full capacity; about 3000 construction factories).
This little wealth world has 20 m construction workers (400 factories) They pop out just under 1 financial center per five day cycle. producing new centers just a bit faster than it produces workers to fill them. Now it has just over 20 million financial sector workers, and its generating almost as much wealth per year as the homeworld... with 15% of the population (150 million versus 1.1 billion). Its almost like I captured an alien homeworld, wealth wise. Given the slow rate of growth of civilian populations, I think one of these worlds is a very sound investment as soon as you can afford to divert the resources. Since you cannot move them, its definatly a diversion of resources-- never put them on a world used for production because you don't want to waste those citizens working finance.