There are three main ways to get data on real estate prices: conducting a study based on standard assumptions (usually comparing with neighbouring properties is factored), using listing prices or using the amount that the legal professional authorised by the Government to record the deed. This legal professional is a Civil Law Notary in civil law countries as Spain or France, or their former colonies. The third way is, assuming there is no tax cheat, the most precise, but it is not universally available, or its geographical detail is not of use; for instance, in Spain the General Notaries Council (www.notariado.org) publishes data by province. The geographical scope is relevant, as the real estate values depend a lot on location, and mixing in the same bag high price neighbourhoods with low price exurbs results in meaningless averages.
In France notaries (www.immobilier.notaires.fr) do publish data with a detailed geographical scope (census blocks). This is good to understand recent activity. But a substantial part of the land has such a reduced amount of sales that data is not representative (or simply does not exist, just think of depressed rural areas with no sales for years). This does not prevent the fact that there is a demand for some kind of data, so it is estimated by a multifactor system, in which listing prices and realtors opinions are factored (www.meilleursagents.com).
In the US the fact that there is a continental size nation with 50 legal systems has led to nationwide portals as www.trulia.com, which estimate prices for a substantial part of the country, even if a large part of the central states, as Texas or Louisiana, are not rendered.
Taking as a reference data from www.meilleursagents.com, www.trulia.com and www.idealista.com for Paris, New York and Madrid, with an exchange rate of 0,72 € by $, and considering that 1 sq m is equal to 10,7 sq ft, you can see that the more expensive areas of Paris (rue du Bac, for instance) are over 14.200 €/sq m, those of Madrid (Recoletos) are around 11.000 €/sq m, and those of New York (Flatiron District) are in the region of the 16.000 €/sq m. Any need for more reasons to understand why the urban fabric of the core areas of successful cities has such an inertia?.