In the 1940s, you could rent a Brooklyn apartment for $20 a month. Today, the median rent has skyrocketed to $3,000.

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Rent in Brooklyn has skyrocketed over the years, as the borough has become more popular.

• A map from the Brooklyn Historical Society used census data from the 1940s to highlight average rents throughout the borough during the time period.
• In most of the areas in northern Brooklyn, such as Williamsburg, Bushwick, and Greenpoint, average rents fell in the $20-$39 range.
• In June 2019, Brooklyn's median face rent hit a record high of $3,000 a month, according to a July report by real estate company Douglas Elliman.
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Nearly 80 years ago, the cost of rental housing in most places in Brooklyn ranged from $20 to $49 a month.

A map assembled by the Brooklyn Historical Society in 2015 used census data from the 1940s to highlight average rents throughout the borough during that time period.

The map shows that in most of the areas in northern Brooklyn, such as Williamsburg, Bushwick, and Greenpoint, average rents fell in the $20-$39 range - that translates to around just $350 to $700 in today's dollars.

The map also shows that the pricier areas of the time, such as Coney Island, had average rents as high as $100 - which translates to around $1,800 in today's dollars.

Read more: 75 years ago, you could rent a Brooklyn apartment for $20 a month

Brooklyn's popularity - and by extension, its price tags - have increased astronomically since the 1940s.

In June 2019, Brooklyn's median face rent hit a record high of $3,000 a month, according to Douglas Elliman's July report on Manhattan, Brooklyn, and Queens rentals.

Some areas that were deemed affordable back in the day are practically untouchable now. Just look at Williamsburg where, as of August 2019, the average rent is $3,317 a month, according to RENTCafé.

Even the more affordable neighborhoods in Brooklyn, such as Flatlands and Gerritsen Beach, have average rents over $1,500 a month.

However, there are some people who are benefiting from Brooklyn's hike in housing costs over the years: investors.

Just consider Greg O'Connell. As Business Insider previously reported, the former narcotics detective built up a real-estate portfolio in Red Hook, Brooklyn, over the years that's now worth $400 million.

Back in the 1960s, Red Hook wasn't the most desirable neighborhood. After industrial buildings and storehouses abandoned the area, the neighborhood was left with empty warehouses and low property values.

In 1967, O'Connell bought his first property in Red Hook for only $22,000, according to his June 2019 interview with Bloomberg. That same property is now worth $2.5 million. O'Connell currently owns around 1.3 million square feet of buildings and 385,000 square feet of undeveloped land in Brooklyn.