Joe has a problem with his car, it makes a terribly annoying sound. He goes to Mechanic A who decides to replace his engine. Problem solved, cost 10000$. John has exactly the same problem (with the same cause), he goes to mechanic B who finds out that a small part of the engine needs replacement. Problem fixed, cost 65$.

Btw, software, unlike buildings or chairs, is extensible and the (technical) desing decisions of the programmer are very important in how extensible it is or if it is extensible at all.