Also, we're talking about land in California, which isn't known for cheap real estate. Even if the central valley might be relatively cheap, the amount of land

Also, we're talking about land in California, which isn't known for cheap real estate. Even if the central valley might be relatively cheap, the amount of land that needs to be acquired in the LA and SF areas, which sprawl considerably, means a good portion of the needed land is likely very expensive for infrastructure. Building something on the 300 miles of I-5 from Tracy to Santa Clarita is probably much easier and cheaper than the last 30-40 miles on either end.