For better or worse, I attended an introductory economics course and some of the ideas have remained with me since, one of which is referred to as opportunity cost. This is a consideration beyond the accounting cost. Neither of these ideas are limited to monetary value; both have broader applications. Put simply, opportunity cost takes into consideration the value of the other opportunities forgone by making a particular decision.
If I own a plot of land and sell it for $300,000 then invest that money in bonds at 4% interest, after three years, I will have gained $37,459.20 according to accounting costs; however, if I consider that a train station will be built five minutes from the plot in two years, significantly increasing the property value and assume that I will be able to sell it for $375,000 in three years to a condominium developer, the first option is actually a $37,540.80 loss according to opportunity cost. Or course, the $375,000 is a speculation and likewise opportunity cost is rarely certain.
For better or worse, I apply this principle to any major and many minor decisions that I make, often to the chagrin of others. In general, time is a much more valuable and scarce resource than money, and thus becomes the measure for many decisions. If I am presently learning 10 abstract learning units (ALUs) in 10 days, after a year, I will have learnt 365 ALUs; however, if instead I use a different method where by I can learn 10 ALUs in 8 days, after a year, I will have learnt 456 ALUs, a significant improvement over the first option.
As a more practical example, consider the acquisition of vocabulary, a major hurdle for most language learners who are serious about learning the language, id est, who are aiming for ability beyond that of the advanced tourist. I have an hour every day to study vocabulary. If I aim for 98% retention after a week, I can learn 15 words in that hour, 0.3 of which will be forgotten after a week on average; however, if I only aim for 80% retention, I can learn 25 words in that hour, since I don't have to spend time on the words that don't stick quickly. Of course, 5 of these will be forgotten after a week, but if you consider the long term, after a year of studying for 98% retention, I will have remembered 5365 words, but by only studying for 80% retention, I will have remembered 7300 words. Again, this is a significant difference. And I don't have to worry about the hard to remember words because I'll see them one week, forget them, see them another week, forget them, see the another week, and finally remember them.
This of course has even broader and more vague applications and that's where occasional problems arise. If I study at this school for a year, my Japanese will improve significantly. "But what if there were a better use of this year of my time?" What is that cliché yet ever so correct and incorrect expression? Ah yes! ignorance is bliss. It is the difference between hearing a sound and enjoying it and hearing a sound and trying to understand why it is enjoyable. People who are given to this sort of thought are well advised to, as Nike says, "Just do it!"
Please apply opportunity cost responsibly.