Well, the power plant still gives +4 per city, so if it borders 4-5 cities, it will also pay itself back in only about 35-40 turns (plus the GE points).
But it does make you think that in more cases, the values really don't make a ton of sense. Say you expect to have about 100 turns left in the game. If your city is getting 35-40 production, then it's going to take you probably 15-20 turns to build a workshop/factory or PP, and then you have to wait another 35-40 turns until it pays itself off, and you're only really getting a benefit from it for the last 30-40 turns of the game, which are probably the least valuable turns to worry about.
So, the question kind of circles back: if you already have enough money to upgrade troops as needed, would it be worth it to buy a factory or PP, or are you probably better to save the money to rush a great person? Or just save it up for the next troop upgrade, or to buy a last troop? This topic definitely has me thinking more about what makes sense to pay off. I mean, it really doesn't make sense to build an industrial zone to get better production in a city, so that you can go build an encampment in a city, so that you can, what, build one troop late in the game? Build a campus? But the other part of me hates leaving districts with buildings not build, and hates spending 50 turns waiting for a campus and library to be built...