This is just a generalization, but it seems to me that when the government sector spends money on infrastructure, the extra bureaucratic red tape plus the perceived deep pockets of the customer tend to raise the price considerably higher than it would be if the customer were a private entity. Sometimes, normal market economic principles seem to go out the window with public projects.
For instance, for the replacement of the I35W Bridge in Minneapolis, the highest bidder was selected. The reason was that the winning bidder addressed the rather odd and subjective criteria in the project calling for maintaining good public relations by keeping the public informed about the progress of the project. The winning bidder convinced the state that they would be the best contractor for handling this sensitive public relations aspect of the project. Other bidders perhaps did not understand this requirement, so they just bid on concrete, steel, labor, and the things that would produce the bridge. If I recall correctly, the winning bid was at least $50-million higher than the second highest bid. It may have been closer to $100-million higher.
The construction bidding was also somewhat of a design contest with each bidder submitting their own bridge design along with their price for building it. I don’t know how you can compare prices when each bidder is bidding on a different bridge. Furthermore, winning the bid depended not only on the price, but also, on the rather whimsical criteria of design style where the designers could push all of the fashionable hot buttons to influence the judges.
The state wanted a bridge that not only functioned as a bridge, but was also a monument to style, fashion, and public outreach. And they wanted as much of this combination as they could get. So it is not surprising that they awarded the project to the highest bidder.