I think that kind of sober assessment is really wise.
Opportunity cost works both ways. The one who goes abroad forsakes what they could have done with the same resources at home. The one who stays forsakes what they could have done abroad. The apostles who stayed in Jerusalem had to forsake what impact they could have made elsewhere. Paul did not take up mission work because there was no possible impact he could have made in Israel, he had to give up influence at home for the sake of ministry abroad. Likewise the man who enters full time ministry gives-up the influence he could have had with his peers in another profession.
I think it'a good to be honest about what one is giving up by going or by staying, but weighing the value of one ministry against the other doesn't seem like it can be done without trivializing the value of a single soul here or abroad. I think that's why people pray for a "leading" or a "calling," a guiding passion for the sort of work they ought to be involved in.
