In the seventies it was about a million a mile to build track from what I recall... but like diapers, it all depends.  Probably more like five million now...depends.
I know if we don`t start paying income taxes and or generating revenue once more soon, like we used to here, capturing millions in slope payroll taxes that otherwise go to Texas, and insisting on a 4% tax floor on our oil production, no matter where it comes from, we will see little then, of good infrastructure projects that opens doors to our interior in so many possible ways.  If it`s not in service lock it out like they practically did the south end some lean years, but gosh FINISH it so it`s available. Three fourths done,..the hardest part.. and there it sits a monument to partisan politics in Alaska, A shame.