In every transfer window, fans around the world ooh and aah about the high price of players, especially in the Premier League. Fees and wages seem astronomical, and a few pundits always try to compare them to prices from the past. They’re usually making comparisons that would drive an economist mad.

Inflation is word we see in a lot of these analyses. But what is inflation? It’s the rate of change of a price for something specific and well defined: a fresh Granny Smith apple, an hour of work by an experienced janitor in Manhattan, or a matinee ticket to a first-run movie theater in Santa Monica. If we want to measure inflation for players, we have to be just as specific. Are we looking at inflation in the price of a 20-goal forward, or a 10-goal forward? A 20-goal forward with two years left on his contract, or one with three? One who’s 25, or one who’s 30? Did he come from Germany, or France? Do we expect him to stay at the club for three years, or four years? Each answer defines a different player.

Maybe we’re not even looking at the price of players at all, but rather the price of a goal or expected goal scored. It might be easier to compare the cost of an expected goal to the benefit it would have for our club; this would be a good way to figure out how much we should be willing to pay. Regardless, we need to have a clear idea of the underlying asset before we start comparing prices.

How to construct those prices is another challenge that’s more complex than it might appear. Transfer fees are only one component of a player’s cost. We need to include wages and agents’ fees, too. Then we have to put all of these components in today’s money, or the money of the year when the transfer took place. That means discounting parts of the cost that won’t accrue in the year of the initial transfer. And finally, we have to adjust this cost for the expected duration of the player’s employment. Then we have the expected price of a given type of player’s services over a specific time period.

Once we’ve defined the underlying asset and figured out how to calculate its price, we can compare prices and estimate the rate of inflation. For example, let’s consider a 25-year-old forward with two years left on his contract who scored 20 goals in the previous season of the Bundesliga and is expected to stay at his new club for three years. If such a player’s total cost was £21 million in 2005, and a similar player costs £35 million in 2015, then the rate of inflation in the intervening decade was around 5%. Of course, we might struggle to find groups of players who are so similar. We can relax the restrictions on specificity in order to add more data points; we just need to be aware that we may end up comparing apples and oranges.

The exercise above allows us to compute inflation rates for different kinds of players. But the question the pundits are implicitly trying to answer is whether these players are getting more expensive in real terms – that is, relative to other things money can buy. So we have to ask ourselves what those other things should be. What’s our yardstick for prices?

If we’re trying to show how expensive players are relative to our own finances, then we might use changes in household incomes or consumer price inflation (CPI) as the yardstick. The CPI index for the United Kingdom rose at a cumulative rate just over 2% in the last decade, so the price of the player above did indeed increase more quickly. But it might make more sense to look at the player’s price relative to a club’s expected annual revenue during the anticipated tenure of the player; after all, we’re not the ones buying him. A big club might have have expected to earn £100 million a year starting in 2005 and £250 million starting in 2015 – thanks to the new Premier League television deal – for a cumulative increase of 10% per year. At that rate, the player above has been getting cheaper.

I don’t expect every pundit to be as careful as an economist when comparing players’ costs. But I hope we can all see the majority of their analyses for what they are: a bit of fun with barely any rigor, just like so many of the soccer statistics we see today.