It's actually better than that. The Shanghai-Beijing line cost ~$40 billion in 2018 dollars. The cost estimates for the California line are in year of expenditure dollars, which means the $78+ billion is amounts that are spent from 2008 until completion in 2033.
The dollars spent early in the project cost more because of inflation, and along the same lines, the dollars spent later in the project cost less because of inflation.
For instance, if we spend half of the $80 billion in the last 5 years of the project, that's something like $30 billion in 2018 dollars even though it's $40 billion in 2028 to 2033 dollars.
