Usually, you try to base it on the recent sales of similar properties nearby. Sale data is more-or-less public information, and there are good aggregators:
I'm curious if the population generally thinks this is a reasonable situation. I can easily imagine a 25% difference in valuation of two properties in the same area with the same number of bedrooms, the same age, same space, and nominally the same amenities. I can also imagine a 25% difference based on some newly opened restaurants, new bus routes, etc., that will not be reflected in historical data.
There is also an enormous subjective-experience component in how people value residential real estate, and enormous variation in details that really do matter and aren't available in real estate data aggregates (like lighting profile, and how intelligently the interior design exploits that lighting profile).
It's so far from what I'd consider reasonable based on my (obviously limited) experience, and I'm genuinely curious how the locals feel about it.
Paris is its own bubble, with prices that are less tied to changing things like new bus routes or restaurants, and more to the perception of what other people pay for it. The historical aggregates usually serve as a bullshit test (if you sell at 25% above historical prices and don't have a very good reason to do so).
It is a given that the price will be very different depending on the state of the property, the quality of the building and its era (1900-1930 flats command significantly higher prices than 1970-1980), whether it's a narrow alley or a large boulevard, whether it has windows on only one side or both sides, whether there's a window in the kitchen and bathroom, and whether part of the building belongs to the public housing agency (among many others). After a while, you start to get a pretty good intuition of how high a property will sell above or below the average.
Another thing about subjective experience is some properties fit certain lifestyles and will be priced accordingly. A flat in the Bastille area (lots of bars, restaurants and night clubs) appeals to people who would pay for the privilege a bonus that a boring father of two (like me) wouldn't consider. Conversely, having an address in the Quartier Latin makes it easier to enter the top high schools, so tiny one-bedroom flats sell surprisingly high (and are then rented out to university students for a far lower price than the purchase price would suggest).
https://www.meilleursagents.com/prix-immobilier/paris-75000/