Chumkiller:
The most simple culvert is a Corrugated Steel or aluminum pipe, or arch culvert. Yes, they are significantly less expensive, but no-one will allow their use in anything but the smallest of streams. And typically the culvert itself is the least expensive element of the improvement.
The culvert of choice for the present is a precast concrete box culvert, which allows natural streambed gravel to be placed in the bottom. For a 60 to 70 foot box culvert, the price is about $80K.including wingwalls and footings, and normally a 2 to 3 month lead time for production.
The bulk of the money is spent on excavation, backfill, streambed remediation and enhancement, traffic control and re-paving the worksite, and other cost elements. The driving factor comes from the prevailing wage rates which are mandated by US & State labor standards, and other direct labor costs. By the time a contractor has sold labor on a project the cost exceeds $50.00 per hour, before Profit.
Welcome to the real world.