Opportunity Cost

💡 What opportunity cost is🔗

Opportunity cost is the value of the next best thing you give up when you make a choice. Your money can only do one job at a time. The moment it goes towards one thing, it is no longer available for everything else it could have done, and the best of those forgone options is the real price of your decision.

It is the most useful idea in personal finance that almost nobody was taught by name. Once you see it, you cannot unsee it, because every dollar you spend, save, invest or leave sitting in an account is quietly turning down an alternative.

🔀 The choice you see, and the one you don't🔗

Say you have $10,000 spare. The obvious cost of spending it is the $10,000. The hidden cost is the road you did not take. Left invested at an assumed 8% a year, that money would grow to roughly $21,600 over a decade. Spend it now and the true cost is the purchase plus that forgone growth, even though the second number never shows up on a receipt.

This is not an argument against ever spending money. A holiday you will remember for twenty years can be well worth its opportunity cost. The point is only to make the trade visible, so the choice is deliberate rather than accidental.

🇦🇺 Where it bites in real decisions🔗

Opportunity cost stops being abstract the moment you attach it to the decisions Australians actually face. These use illustrative assumptions, not predictions, and they ignore tax, which shifts the exact numbers.

The offset-versus-invest question is the classic one, and there is no single right answer: it turns on your interest rate, your tax position and how you handle risk. Our guide to paying off the mortgage or investing works through it properly.

👻 The cost of doing nothing🔗

The sneakiest opportunity cost is the one attached to inaction. Leaving a large balance in a low-interest transaction account feels like the safe, responsible choice. It is not free. That money is turning down both the return it could have earned and, worse, it is quietly losing buying power to inflation.

🎯 The essential: "Doing nothing" is still a choice, and it has a cost. Cash that is not needed soon, sitting where it earns little, pays the opportunity cost of the growth it forgoes and the inflation it absorbs. A high-interest savings account or an offset is often the smaller sacrifice.

🧭 How to actually use it🔗

The practical version of opportunity cost is a single question: what is the next best thing this money or time could do? You do not need a spreadsheet. You need to name the alternative and decide, on purpose, that your chosen path is worth more to you.

• For a big purchase, put the price through a rule of 72 or a compound interest check to see what it could have become. Then decide if the thing is worth that.
• For a lump sum, line up the two or three real options side by side, including offset, super and investing, rather than defaulting to whichever is easiest.
• For recurring spending, do not agonise item by item. Set a budget once, and let the trade be handled by the plan instead of by willpower at the checkout.

⚠️ Where people get it wrong🔗

• Confusing it with a sunk cost. What you already paid is gone and should not weigh on today's choice. Only the forward options matter. Our guide to the sunk cost fallacy covers the trap in full.
• Ignoring risk. The alternative with the higher expected return usually carries a wider range of outcomes, including bad ones. A forgone 8% is not a guaranteed 8%, so compare like with like.
• False precision. The exact dollar figure is an estimate built on assumptions. Use it for the direction of the decision, not as a promise about the future.
• Analysis paralysis. Weighing the opportunity cost of every tiny choice burns the very attention you are trying to spend well. Reserve it for decisions that are large or hard to reverse.

❓ Frequently asked questions🔗