Question

Cost is most plainly visible when spending more money on one thing means that less money can be spent on another thing.

Answer 1

This sentence means that the true cost of spending more money on a particular thing is that you have less money to spend on other things. In other words, when you increase your spending on one item, you must compensate for that by reducing your spending on other items. Therefore, the cost is not just the monetary value you spend on one thing, but also the opportunities you lose by not spending that money on other things.