Dry cleaning is surprisingly profitable having a 200% markup at retail and 25% profit on the average transaction. A chain often has a single facility that actually does the work with the rest just being a drop-off/pick-up location. So each pick-up/drop-off location enables more revenue with a relative modest increase in price (1 or 2 salaries, rent, and shipping to and from central facility).

A similar set up on Bizbuysell.com claims cash flow of $287,000 or $1,271,458.01 in 1975 dollars. As a back of an envelope kind of calculation it appears that an east side apartment goes for $45/square foot in the 1970's (compared to $250/square foot 10 years later or $1,070/square foot today). So, it appears that the story is plausible, taking home six figures and only having to spend thousands on the apartment.

Citations:

http://www.examiner.com/article/buying-a-dry-cleaning-business-is-there-money-to-be-made

http://www.bizbuysell.com/dry-cleaners-for-sale/

http://blog.elliman.com/market-intelligence/change-is-the-constant-in-a-century-of-new-york-city-real-estate

He owned 7 dry cleaning stores at a time where the cost to rent/purchase prime NY real estate cost about 5% of what it does today with inflation making today's money 4-5X his. So, his money would be worth 1/5th whereas property would cost 1/20th. He'd be able to afford 4X the square footage of a deluxe apartment(in they sky-eye) more than a guy with 7 dry cleaner businesses today. It definitely seems reasonable.

I've no idea what "owner of dry cleaning chain" might imply in income and/or assets but the New York Times is able to help with the price of apartments.

This gives you some idea of what might be found : http://query.nytimes.com/search/sitesearch/#/price+east+side+apartments/from19721010to19751010/

Why would you not just specify a hypothetical income? An owner of dry cleaners can make $500k per year or lose $100k per year. Silly question.