Leading the pack is likely to be the new Four Seasons Tower, which could see its three, gold-plated penthouses on its 61st floor each fetch upwards of $20 million, according to preliminary price estimates.

That means the Back Bay tower’s top floor may reap a total of $60 million to $70 million in sales. That figure would dwarf the $37.5 million that the 60-story Millennium Tower — now under construction in Downtown Crossing — is seeking for its top floor penthouse suite.

Construction recently kicked off on the $750 million Four Seasons tower taking shape near the Christian Science Plaza, with an opening day pegged for 2017. It will be Boston’s tallest residential building.

“This is absolutely like nothing Boston has ever seen before,” said downtown Boston broker Tracy Campion, whose Campion & Co. is selling the Four Seasons condos. “I am absolutely sure they will set records.”

So what exactly will it cost to buy one of the two-story penthouses that will crown the new tower?

Campion declined to offer up an official pricetag but indicated that a “ballpark” estimate of $3,000 per square foot was not off the mark.

Other brokers not connected with the new tower offered similar price estimates for the Four Season condos of roughly $2,500 to $3,000 a square foot.

With each unit offering 8,000 square feet of living space atop Boston, that would put the penthouses in the $24 million range.

That’s practically double the previous price record set by a condo at the posh Mandarin Oriental, also in the Back Bay, which fetched $13 million.

The Four Seasons penthouses will have a number of features — for instance, a “huge” staircase, an elevator that opens into the unit, 14-foot high ceilings — that will likely make them a hot item, especially among the deep-pocketed international investors who have begun to flood the Boston market.

And while the prices are likely to come as a shock to the Boston area, jet-setting buyers used to stratospheric New York and London values are not likely to bat an eyebrow, brokers say.

In fact, $2,300 to $3,300 a square foot is simply considered “luxury” in the New York market, with “uber luxury” in the Big Apple now priced at $5,000 a square foot, according to Otis & Ahearn, a downtown Boston luxury condo marketing and brokerage firm.

“We are not used to those numbers but those are numbers we are going to start to see,” notes David Crowley, director of sales and marketing at Raveis Marketing Group. “They are building these properties to appeal not necessarily to a local market, but to international buyers and investors.”

“International tastes are driving a lot of the finishes, layouts and amenities,” Crowley added.