I read the article you cite and the section in question is 4.5 blocks long. And the project included drainage, street lights on the far side away from the bike

I read the article you cite and the section in question is 4.5 blocks long. And the project included drainage, street lights on the far side away from the bike lane, paving the whole road, sidewalks, etc. The Seattle city-wide average was $2mm/mile. Very informative article, and it is completely different from what I thought I'd read based on your summary.