I don't understand your argument. You get 24 fp for free for playing the game. Those fp can be used anywhere. Donating them to your traz means not using them somewhere else. Using your space for a traz means not using it for something else. The term for this is opportunity cost.
I'll give you an example of what I mean by free. In 2001 I was paying rent. At that time for a little more* I could make mortgage payments. So I bought. Either renting or buying, what I am spending the money on is the roof over my head. However, in a few years, as a byproduct of spending the money putting a roof over my head, I'll own a house. The house will be free.
Since I already had the free FPs, coins, medals, and diamonds and I was going to spend them on something, the Traz is free in respects to FPs and space.
Dunno why you wanna drag Opportunity Cost into this, but if'n you do, I reckon ya gotta invite it’s partner, Return on Investment.
RoI of Traz: The most efficient accumulation of unattached troops.** This is also the OC of not building Traz. I’m ignoring happiness, I don’t see it as a major factor in this argument.
I’ll let you figure out the other two (RoI of building something else, OC of building Traz), then we can compare.
*Three years later the rent was higher than my mortgage payment.
**I think GE is the only way to accumulate unattached troops on a scale comparable to Traz. I think you can get 24 mundane troops of your Era per week if you get all the way through. Maybe a few rogues and champs as well. Traz is much more efficient in that the troops are free, it produce more troops, and you control the types of troops collected.
p.s. If you want, I’ll post the OC/RoI analysis of renting versus buying the house. Boring no end. A.