worst thing you can do in FoE: think about RoI. In FoE, RoI is nothing!!! all that matters is have the GBuildings at a high level as soon as possible. is not the same level up your hagia up to 80 in 30 days to in 10 days. while in the first case you earn 480 FPs from day 30 to 40, in the second case you earn 1440 FPs from day 10 to 40. a difference of 960 FPs. and as soon as you reach a high level, you can continue with another GBs. it doesn't matter the cost of the GBs but how fast you level them up. wich is the difference between level up 10 levels by spending 1M of fps in 20 days to 10 levels by spending 1000 fps in 20 days? nothing! the amount of time is the same!!! time! time is what matters!
RoI is all about time - if RoI doesn't allow you to calculate the shortest path to a particular goal for some reason it's because you're ignoring other costs or payoffs (such as alternative uses of space/game time/FP investments).
Let's say you have a modest production of 500 FP / day in a newish city that's been playing events and working on arc up to 80 so far and your goal is to have Zeus, CoA, CdM, Traz to 65 - because past that point the incremental gains while nice aren't so important that you need them on any particular schedule. You already have access to a 1.9 which will feed you all your blueprints for free, and take all your spots efficiently. You have a reserve of X FP that's good enough for your engagement with that thread and you're not going to raise/lower that in general. So you have 500 FP/day. If you take the direct path:
Zeus 65: 12503 FP
CoA 65: 13323 FP
Traz 65: 20421 FP
CdM 65: 16642 FP (including the bonus collections after each level)
Total : 62709 FP
The straightforward path to get there: ~125 days.
Now we can talk optimisations to this. The most simple one is "well I'll do CdM first, because then it'll make more FP sooner and take time off the others".
So 16642/500 = ~33 days. Then the remaining 46067 will be at 539 FP / day = ~85 days. You saved about 7 days compared to doing CdM last, hurray (actually would be a bit better than that because while levelling CdM you'd be gaining speed; but this is a close enough approximation)
A more complicated optimization might involve early traz levels for extra GBG fights that make you extra FP and that might be even more impactful to a point than the early CdM levels. But there's really no way to model that without accounting for a particular player's situation.
Now let's say we try to insert a Cape to 60 before doing any of this - it wasn't part of your objectives so it's both increasing your FP production as well as the FP demand to reach your goal. It's the fastest returning building of any available. Cost of Cape->60 : 14076 FP (less bonus collections). That'll take you ~28 days.
Then you'll do CdM to 65 at 560 FP / day : 16642/560 = ~30 days.
Then you'll do the rest at 599 FP / day : 46067/599 = ~77 days.
In total your 28 days on cape saved you 11 days on what you really wanted. So your goal would have to be something more to justify doing Cape before your real objective. Now if you tack on a few more buildings (I want a TA and HC and CF and BG and whatever else), cape will surely pull ahead and have saved you time. The math for hagia is not so clear however. Especially if you start throwing in opportunity cost of "well it's not hagia vs nothing - it's hagia vs these event buildings" (which will vary from player to player)
You view things from the perspective of someone who's already past the point of having defeated all reasonable usage of FP. Any marginal return you "may as well" because you make so much FP that bad decisions wouldn't really show through anyways and you're out of good returns. Past a point having a larger bank is just a matter of laziness.
It's become rather easy in game to get to that point of late. It's not because of your Hagia you're rich. But you're not wrong for having one either. It also doesn't mean everyone should build one.