Take the new sewer under the Thames. Absolutely vital to sustain the growth of London, but there's no money in poo. Without it though, we well have The Great Stink 2.0. The growth in GDP that would follow the growth of London will eventually pay for the sewer, but something designed for the next couple of centuries isn't going to pay for itself within a couple of years.
The jobs created, though will create more jobs and revenues etc.