Why are internal sources of recruitment considered to be more economical?

There are two types of internal recruitment – 1) Transfer 2) Promotion. This process is fully economical as it saves time and money. Whereas in the external process, they have to visit the campus or display advertisement and selecting some candidates, thereby spending a lot of effort, time and money. Moreover the new employees need to be trained according to the system so this process is a bit expensive and time consuming. Hence internal sources of recruitment are considered to be economical.