Most big projects of this nature come in over budget. The reason is the incentives of the politicians coming up with budget estimates.
If they come up with a realistic (high) estimate, the project doesn't get done. That's a political loss - their cronies don't get paid, and they don't get to put their name on a big project. If they come up with an unrealistically low estimate, their cronies get paid and they rarely get voted out of office.
If they come up with a realistic (high) estimate, the project doesn't get done. That's a political loss - their cronies don't get paid, and they don't get to put their name on a big project. If they come up with an unrealistically low estimate, their cronies get paid and they rarely get voted out of office.