Opportunity cost and investing
If you buy $1,000 worth of stock in a company, the basic cost is pretty obvious -- $1,000. However, you might also want to consider the possibility that your funds could be put to better use, and you'll be able to calculate the opportunity cost of your decision in retrospect.
If the stock you purchased remains perfectly flat over the course of a year, it might not bother you much. However, if another stock you were thinking of investing that $1,000 in goes on to have a great year and triples in price, you'll likely be upset by the opportunity you lost by investing in a company that underperformed compared to the alternative option you were considering.
The alternative stock would have yielded a profit of $2,000, while the stock you actually bought yielded zero profit. Your investment didn't lose money, but in retrospect you can see that there was an opportunity cost of $2,000 for not buying the other stock instead.
In the stock example detailed above, having to pay $1,000 to acquire the stock is the trade-off. The trade-off to acquire either of the stocks in question would have been the same, but not going with the better performer had an opportunity cost.