I've added a separate entry for the original 1877 bridge:
I think I see where the $20,000,000 figure came from. $20M in bonds were sold for the construction of the Cincinnati Southern Railroad, but that was for the construction of the 350 mile rail line.
From bridgesandtunnels.com "Construction was completed by the Keystone Bridge Company of Pittsburg for $750,000 and was regarded as one of the cheapest spans of bridge-building in the West. It was the longest truss span in the world. The span was the seventh bridge built over the Ohio, and the third at that point."
I would like to see the figures on how they arrived at $20,000,000 cost. That sounds way too high for 1922 and imposable for the earlier date. I would guess the 1922 price at about 1/4 that.
I believe this bridge was actually built in 1922. The substructure may date to 1877, but the superstructure is far newer than that. Wikipedia claims the bridge was "extensively modified" in 1922 which I believe means replaced. http://en.wikipedia.org/wiki/Cincinnati_Southern_Bridge