As the European transfer window reopens this month, the soccer world’s headline writers and rumormongers have once again begun their biannual ritual of sticking price tags on soccer players. Clubs may be coy about how much they’re spending on Italian striker X or Spanish midfielder Y, but a ballpark figure almost always leaks out.

But here’s the problem with the seven- and eight-figure sums fans love to throw around: They mean little on their own. The transfer fee, easy as it is to discuss, never provides a clear indication...