https://connect.ncdot.gov/letting/Division%2014%20Letting/Fo...
I'd also add that any sort of stream crossing will have pretty stringent restrictions for working in the wet - you're limited to time of year, weather events, need to maintain safe overflow at all times, fish passage, etc - all of these add cost, and limit how many people will bid on the project (they need to dedicate schedule time to the work - can't push it off a month because of delays on another project, or it'll take them out of work period).
I couldn't tell if it also requires the maintenance of road traffic at all times - if it does, even maintaining one lane at a time (with automated signals) would easily add $100k to the price (culvert needs to be removed and installed in sections, then extra work to join them together, much smaller paving operations, etc etc)
It looks like the kind of project that might work better cost wise if you could lump say 10+ replacements together and have a 3-4 year long project.
The other 2 projects on HC Rd were controlled with auto signals and occasional, brief closures. Unsure of time-frames, as they were already underway when I arrived.