The more I think on that 'unit cost' study, the more I think the whole premise is stupid, and you've got the right angle with the 'marginal cost' approach. How much the last 10 units cost is irrelevant, compared to what the next 10 units are going to cost. Adding the first 10 lines to I-15 was cheap, because it was (mostly) rural land. Adding the next ten lanes is going to be very expensive, because it's either going to require: a) condemning hundreds of acres of urbanized land, or; b) decking the freeway. So how cheap driving was/has been is irrelevant.
The kind of business thinking this entails isn't relevant for infrastructure investment. Infrastructure is much more about making major capital investments, ie: investing in a new factory. When you do it, you do so not just for today's needs, but for room to grow. It might be more economical to go with the cheaper alternative now, but in the long-term, the cheaper alternative is inferior.
Take this example:
