When someone in the ancient world is said to have a fortune of x talents, y drachma or z denari - or if a medieval fortune is described as being so many gilders, doubloons, or ducats - how exactly can an historian equate this amount of money to modern day purchasing power?
Do they tie it to a common commodity like the price of bread? Or do they just take the current value of the gold or silver in the coins in question?
Is such a comparison even meanful since much of what we can buy today would be inconceivable to someone in the past?
Your final point, that it's difficult to judge because so much of what we buy now is so different from the past, is accurate.
The most common thing I've seen scholars do is indicate in a footnote the approximate daily/weekly/annual earnings of an average member of the society being discussed. They'll also make a point to explain how valuable something is in terms of the state. For example, if a Greek polis opened up a new silver mine worth X talents of silver per annum, they will note how many talents would pay for the construction and crewing of a trireme.