As you edited, Seinfeld wasn't a two bit comedian. He was able to buy his parents a Cadillac Fleetwood from one well-paying gig. This was season 7 in 1996. At the time, that vehicle had an MSRP starting at $36,000. https://www.edmunds.com/cadillac/fleetwood/1996/review/

A 1996 piece from the New York Times interviewed rental agents and in places near central park (Upper West and east side), the rentals were as follows:

Through the first six months of this year, according to a survey conducted by Feathered Nest of about 1,500 regulated and market-rate apartments -- many of them sublets in co-ops and condos -- rents for studios on the Upper East Side averaged $1,361, 3 percent more than the $1,317 only seven months ago. On the West Side, studios averaged $1,459, up 8.7 percent from $1,342 at the end of 1995. One-bedroom apartments on the East Side are currently averaging $2,067, up 4.9 percent from $1,970 last year; while on the West Side, one-bedrooms averaged $2,037, up 1 percent from $2,019 in December. The survey looked at a total of 343 one-bedroom apartments on both the East and West Sides.

For two-bedrooms on the East Side, rents are averaging $3,315, compared to $3,281, a 1 percent increase. The average two-bedroom on the West Side is up 6.8 percent, to $3,406 from $3,190.

And the three-bedroom market, according to the survey, which looked at a total of 48 apartments, is showing startling disparities. Three-bedroom apartments on the East Side are actually down 10.1 percent, to $4,687 from $5,214 at the end of last year. But three-bedrooms on the West Side are up a whopping 44 percent, to $5,910, compared to $4,100 in December.

https://www.nytimes.com/1996/07/14/realestate/market-rate-rents-in-manhattan.html

Seinfeld's apartment was a 1 bd, so around $2000-$2100 a month. maybe during the earlier years, it would have been a stretch, but if you read the quote, 1996 saw a significant increase in rent as a percentage in 7 months for a 1 bd unless you were on the upper Westside. The early 90's were a time of economic recession until July 1991, so it still could be affordable. We just don't know how much he was paid as it's obviously not a steady income.

I think you are supposed to suspend reality for the rest of the characters, like all but maybe Ross and Chandler in Friends. They could also be rent controlled, perhaps in Kramer's case.

Here is a summary of housing in 1996 from the City. On page 10, it goes over rent controlled units:

Changes in the Rental Inventory by Rent Regulation Status Rent controlled units in the City numbered 71,000, or 3.6 percent of occupied rental units, in 1996. They housed 127,000 individuals, or 1.8 percent of the population. The number of rent controlled units declined by 31,000, or by 30 percent, from 101,000 units in 1993. In 1996, 1,052,000 rent stabilized units, occupied and vacant together, the largest single category of rental housing, comprised 51.9 percent of the 2,027,000-unit rental stock in the City. This represents a slight increase of 4.3 percent since 1993, when the number of rent stabilized units was 1,009,000. Rent stabilized units housed 2,441,000 individuals, or 33.8 percent of the population, in 1996. The number of in rem units declined by 30.4 percent, or from 36,000 to 25,000 units, between 1993 and 1996. This dramatic decline resulted mostly from HPD's effective disposition policy to return the units to the private sector, as part of the agency's comprehensive anti-abandonment policies. In 1996, there were 576,000 rent unregulated units in the City: 500,000 in rental buildings and 76,000 in cooperative and condominium buildings. Between 1993 and 1996, the number of rent unregulated units in cooperative and condominium buildings declined considerably from 95,000 to 76,000 units, or by 20.6 percent.

https://www1.nyc.gov/assets/hpd/downloads/pdfs/about/hvs-summary-1996.pdf

In 1996, it says around half of all stock were rent stabilized.

Here is the median rent stabilized price for all Manhattan stock from page 17. So, maybe George or Elaine. Kramer had a pretty good location, as you mentioned.

The median contract rent of rent controlled units in the City was $428 in 1996, $172 less than the citywide median of $600. Compared to the proportional distribution of renter-occupied units in the City by rent level, many more rent controlled units were low-rent units and fewer were middle- and high-rent units. In this rental category, almost three-quarters of the units rented for below $600: 43.4 percent for less than $400 and 29.9 percent for $400 to $599. Most of the remaining units in this category rented for $600 to $999.

As you can see, it's a lot cheaper than the average from the NYT article.

Further, on page 20, it goes over the rent-to-income ratios with the following:

In 1996, half of all renter households paid 30.0 percent of their income, the standard affordability measure, or more for rent: 22.4 percent paid between 30.0 and 49.9 percent, and 27.9 percent paid 50.0 percent or more. On the other hand, of rent-subsidized households, 78.6 percent paid 30.0 percent or more of their income for rent: 20.2 percent paid between 30.0 percent and 49.9 percent, and the remaining 58.4 percent paid 50.0 percent or more

Breaking it down for Manhattan and Queens (They didn't separate) we get:

In Manhattan and Queens, where the median gross rent-income ratios were 27.8 percent and 28.2 percent respectively, 54.0 percent and 53.2 percent respectively of renter households paid less than 30.0 percent of their income for rent, while 16.0 percent and 13.9 percent respectively paid more than 70.0 percent.

So you can see, it actually isn't terribly unaffordable for 40% of them. For 58.4% of rent controlled renters, they paid over 50% of their income to rent.