Unlikely you will find a hard figure for the 1965 cost, but consider what NCRA spent (not what it cost to actually work on the line, which is probably a small part of the bill) to bring Lombard-Windsor up to minimally freightable condition. Then think about essentially building a new railroad (maybe a few of the bridges could be repaired instead of replaced) in steep, unstable terrain with poor road access. After doing a huge EIR (educated guess: $30mil minimum for the EIR alone, only for the line N of Willits). And after of course rebuilding the rest of the line from Windsor to Willits -- no small job there either. Whatever it cost SP in 1965, add an order of magnitude - maybe two - and double it. Total bill (for everything north of Windsor) close to a realistic estimate for the pipe-dream line over the mountains wouldn't surprise me.

Did SP ever make money with the NWP, or did maintenance (and periodic rebuilding after big floods - 1965 wasn't the only one) and operation more than eat up the revenue? And remember, that was with a lot of lumber shipments (which no longer exist) and no real competition from trucks and modern barge/other seaborne shipping until the 1960s.