What makes a cost relevant to a specific decision?

Answer 1

Assuming you have a specific (limited) amount of money, you can spend it on only a limited amount of uses. You need to determine whether spending your resources for that (meaning you can't spend them for something else which might also be nice) is worth it.

Answer 2

In what field?

Cost mostly is the deciding factor if the decision is a yes or a no.

A doctor wants to cut of my leg.
Two options
1) Yes-- costs me a leg.
2) No---- Costs me my life.

Option 1) sure looks the best.