By 'return on investment' I mean most good done per unit time, or maybe per unit energy. For example, I could spend an hour every day picking up garbage off a highway, or I could work an extra hour at my job and donate the money made during that hour to a charity. Presumably there would be an hourly wage in that scenario which is high enough to justify working the extra hour as opposed to picking up litter, (assuming the charity the money is donated to is some hypothetical perfect chairty) if the goal is to do as much 'good' in the world as possible with that hour. Similarly, there would be a wage low enough that it would be more worthwhile and beneficial to just pick up litter. Or maybe the true best option is to spend that hour studying or learning some skill which would lead to making more money in the future so that the same hour investment would be worth far more... but then again maybe money or is actually 'worth' more now, as in it will do more good now, than it will in the future, and that's a strong enough effect to offset the increase in earning potential down the road. For example giving $50 to a homeless person who's one day away from starving to death may be more 'valuable' than spending that money on a say a textbook for college so that in 5 years you're making $70k/yr and give $2000 to the Catholic church or whatever. Maybe poor example. That's the sort of mental calculation I'm thinking of though, if it makes any sense.
Essentially investment can represent time or moneh or energy, and return represents some tangible 'positive' impact (defining what constitutes positive impact is a whole other thing too). The idea is to maximize that ratio to the best of one's intelligence, because doing anything other than that would be negligence or 'bad faith' (you COULDVE done more... think 'I held the door open for someone so my good deed for the day is done')
Excessive wordiness.. high estrogen