I saw a meme go by about how the modern jumper-prevention nets on the Golden Gate Bridge cost a third as much as the inflation-adjusted cost of the bridge itself. This seemed unbelievable, but it actually mostly checks out. (The original meme overestimated the cost of the nets, but I list the corrected value here)
The bridge cost $35 million in 1930-1937 dollars, which works out to ~$800 million in today's dollars. By comparison, the replacement of the eastern span of the Bay Bridge in the 2010s cost $6.5 billion at the time, or $9.2 billion today.
What's the difference?
Is it labor costs? There is some difference, but not nearly enough to account for the difference in costs.
In the 1930s, workers on the bridge earned $4-$11 per day. This works out to $12-$32 per hour today (assuming 8 hour workdays). In comparison, the SF Ironworkers Union pay is currently $22-$38 an hour (~5th-95th percentile).
Is it the cost of steel?
No, it's not that much more expensive now than it was in the 1930s. It looks like the current price of steel is about 1.4x the inflation-adjusted cost of steel in the 1930s. However, this appears to be some kind of pandemic supply chain issue; the price in the 2010s was very close to the inflation-adjusted 1930s price.
Is it worse safety practices in the old days allowing for more efficiency?
The Golden Gate Bridge was famous for strict safety protocols and a then-extravagant $400,000 safety net to catch bridgeworkers who fell. The construction of the bridge had a mostly excellent safety record, with only one death until close to the end of construction. However, close to the end of construction, some scaffolding collapsed through the safety net and led to the deaths of 10 additional workers.
In comparison, no workers died during the retrofit of the Bay Bridge's eastern span.
I'm guessing the radical difference in cost we see is some mix of better handling of occupational hazards (yay!), better handling of environmental issues and side effects (yay!), and far more bureaucratic meddling, paperwork, and pointless restrictions (ugh).