Now if you multiply these hypothetical two or three weeks by the hourly rate of someone who can learn, you'll get the cost of replacing the author with another employee.
My point is that the cost could've been much smaller.
In my eyes, you neglect the opportunity cost of "using one tool for everything": You end up with a solution that is much harder to mantain, since the wheel will have been re-inventend many times over and sprinkled with bugs along the way. Given a problem of fixed complexity, at some point costs will rise when fewer tools are used.
In summary: There is a "too much" and a "too few" when it comes to the number of tools used in a project. Where in that spectrum the OP is falling, we can't possibly know without knowing more details. Maybe the cost could have been smaller, maybe they hit an optimum and costs couldn't have been reduced by fewer tools (or more tools).
My point is that the cost could've been much smaller.