It’s talking about cost mainly in terms of land value. If the parking spot was lost and had to be rebuilt in a similar location, what would that cost? In the centre of a city the cost of tarmac isn't going to be the dominant issue!
Using city population for a per capita calculation seems perfectly reasonable as long as that’s understood, even in most tourism cases. For instance it’d be a perfectly reasonable economic calculation to look at the tourism income brought into a city per head of the resident population, it gives an idea of how valuable that is to an ordinary person, even if they don’t work in the industry themselves. The calculation being made here is similar.
This only dramatically falls down where the land value is dominated by car-borne tourism value, for instance for a stop off on a long distance road route, reducing parking spaces by 50% might well reduce income by 50%. But that doesn’t apply in the central areas of most cities.