“Cheap” is subjective. The troublesome gestation of the Circle Line did result in significant cost overruns, with the end price tag of the line probably closer to $10 billion.
But why I say it’s subjective is because of the Downtown Line, which is only 20% longer but cost double the price at $20 billion. Still, sacrifices were made to keep costs down, and some of them may not have been very optimal, especially considering their more polished equivalents along the DTL.
This isn’t the first time I’ve gone on about the Circle Line, really, but it’s a continuous series of errors and structural failures that got us to this point, so I don’t believe the finger can firmly be pointed at a single politician, civil servant, or group of engineers.