This is another instance in which I go against the prevailing opinion: I buy FPs pretty much only when an Event Step or a Quest requires that I do so, which is why in the four servers on which I am in what I suppose would be considered mid-game (ME, PME), fps still cost me less than 10k each.
Now, that said, it isn't difficult at all to set up your city to produce crap tons of gold per day. Quest rewards can be gold. GE rewards can be gold. Many of the Event reward buildings produce gold. Shrines of Knowledge produce gold. Motivating/Polishing now produces gold based on the Age you're in rather than the straight 20gp it used to produce. Housing, of course, produces gold. You can use Tavern silver to buy gold production boosts. And then St. Mark's Basilica provides perhaps the highest percentage boost to production of any GB in the game GB level for GB level.
Heck, I'm probably leaving out several other ways to produce gold.
So yes, I would think that it can certainly be a viable strategery with moderate effort on the part of the player.