No GB is worth it if all you're going to count is the FPs you may have to put into it.
Not true. Tower of Babel provides headcount and goods in exchange for those FPs. Huge Sofa provides happiness. Deal Castle provides medals and city garrison attack/defense. For these GBs, the decision is "Do I want to invest FPs in order to get these other bonuses?"
The only thing Cape Carnival does for you is refund the FPs you put into it...
slowly. Because it provides no other benefit, and takes up land space that could be used for other purposes, the only meaningful question here is "Am I willing to play this game long enough to collect more FPs from my Cape than I invested, and during that waiting period, am I comfortable consuming that land space waiting for that ROI and sacrificing (think opportunity cost) another potential bonus?"
My personal opinion: Cape is only good as a very long-term investment, and I'd rather use those FPs on just about any other GB. I mean, think about it like this:
Cape: "Hey man, spend a bunch of goods to build me and then pump some FPs into me! C'mon!"
Me: "Uh, o.k., what's in it for me?"
Cape: "Dude, I'll totally give you back your FPs."
Me: "You will? Awesome! How long will that take?"
Cape: "LOL like years, bro! But hey, I look SO cool!"