The number of commercial leases with tenants agreeing to pay more than $100 a square foot has increased to at least 31 this year from three in 2002, according to estimates by firms that track city real estate prices — fueled by record-low vacancies, a booming economy, and old-fashioned conspicuous consumption.

While private equity firms and hedge funds make up the majority of top-dollar payers in class-A space, they are increasingly being joined by other types of firms that several years ago might not have fathomed paying $100 or more a square foot.

“It used to be there were only sporadic deals,”a first vice president in the New York City office of CB Richard Ellis, Ben Friedland, said.

Now the number of landlords demanding triple-digit per-square-foot rents — and tenants willing to pay them — has spread modestly from the Plaza District on Park Avenue north of Grand Central Terminal, where the average asking rent is close to $80, to areas like Lexington Avenue and Midtown.

Cushman & Wakefield said the firm is the agent of a building on 40th Street and Park Avenue in which the upper floors have asking prices that surpass $100 a square foot.

Those rents used to be imaginable only at buildings like 9 W.57th Street or the General Motors building at 767 5th Avenue, and even then almost exclusively the province of private equity firms for whom enormous rents are a “rounding error,” in the words of Mr. Friedland.

That’s changing, real estate industry sources say. Tenants who have joined the $100 club include cement maker Cemex at 590 Madison, recruiting and consulting company Berglass Grayson at 399 Park Avenue, and technology firm Unaxis at 767 5th Avenue. Comedian Jerry Seinfeld has office space at Carnegie Hall Tower at 152 W. 57th Street that costs at least $100 a square foot.

Law firm Akin Gump Strauss Hauer Feld LLP announced this summer that it would pay more than $100 a square foot for 203,000 square feet of space in Douglas Durst’s Bank of America Tower at One Bryant Park — one of the records for so much space at so high a price.

“You just never saw things like that before,” an executive director at Cushman & Wakefield, Alexander Chudnoff said, adding, “that’s a huge, huge diversion from where law firms have been.”

The increase in leases being signed at or above the $100-a-square-foot mark comes as the overall asking rate in Midtown increased to around $62 from around $54 a year ago — spurred by real estate statistics showing that city vacancy is at an all-time low.

Until very recently, the $100-a-square-foot threshold had been one component of finance-industry marketing tactics to retain top-shelf employees and assure investors that their billions are in competent hands.

“It’s a recruiting tool,” Mr. Friedland said. “You want to keep the top people. They’re working very hard.”

The rent deals often include an allowance for “build-out,” so some of the rent money may fund fancy kitchens, marble bathrooms, and elaborate wood or glass finishes.

“The image they want to project is the best of the best,” Mr. Friedland said. “Everyone is cognizant of where their friends and competition are,” Mr. Friedland said, adding, “They’ll say, ‘check out my gym, check out my kitchen.'”