Alternative form of why buy a book when you can join a library

Do not expect somebody to make a costly commitment if it will give them nothing they do not already have; used especially as a warning against having sexual intercourse before marriage.

Synonym of if all you have is a hammer, everything looks like a nail

Synonym of if you find yourself in a hole, the first thing to do is stop digging

It is easy to lose sight of one's initial objective, becoming caught up in subtasks or in tasks only tangentially related to the original goal.