If you want to put the Panama Canal through Boston it's going to cost astronomically more than $24bn — the logistics (not to mention the requirements and processes of the time period) are entirely different.
The grandparent poster compared the cost of the big dig to the cost of the panama canal to in an attempt to show that the big dig was overly expensive.
This parent poster is pointing out the fallacy by hypothesizing that the Panama canal in Boston would be way more expensive than the Panama canal in Panama, thereby showing the error in the grandparent post's reasoning.
