I can't find what the cost for reconstruction is on average, but I found the average cost for new construction and ROW acquisition is. We could rebuild 20% on all existing roadways in the United States, from scratch, with that much money. Or, we could give just $1.3 billion of it to Nebraska and fix every single road in the state and save the country $38.7 billion.
FWIW, Delaware is the most expensive state to build a new lane-mile at $80,320 in 2014. That means in Delaware, you could build a new 4-lane highway for 65 miles for the approximate cost of building one mile of wall.
Nebraska was 49th at $6,835 per lane-mile.
https://midwestepi.org/2017/05/03/what-are-road-construction-costs-per-lane-mile-in-your-state/
https://www.fhwa.dot.gov/policyinformation/statistics/2013/hm60.cfm