It’s cheaper in the sense that it is faster to write and maintain those 20 lines of code. Because someone has to evaluate the library, understand it well enough to actually call it and then make sure it stays up to date. And often there are a few lines of code to translate your data into a form that the library requires etc.
Plus for every developer to come, one call to an external library usually also means 30 pages of documentation to trawl through if they ever want to change anything, 29.9 of which is completely irrelevant to whatever your narrow use case is.
That's the real cost. The size of the code means absolutely nothing.
