The generally accepted measure is an income above the national average.
Details
In Germany, a "good" salary is €40,000-43,000 after taxes per year (= about €3,400 per month). In Berlin, mid-level "white collar" workers are paid an average of €28,000-40,000 per year, which is called "not enough". At the same time, in the capital, the average salary and prices are higher than in the country as a whole.
Mid-level specialists in London earn from €35,500-47,000 net per year. This is roughly equivalent to a good salary in the British mind. In the capital, the average income is 23% higher than in the country.
In France, a "good" income is considered to be €3,200 per month (net) for one person. However, in Paris, you will need €3,400 per month to live comfortably and cover your expenses.
The average net salary in Spain in 2024 is €1.8 thousand per month, “for a comfortable life” - about €2.7 thousand. And for Madrid... €3,185. Mid-level specialists in the capital can count on €26-38 thousand after taxes (per year), that is, significantly above the average level.