Less than a month before the year's last Grand Slam gets under way at Flushing Meadows, ticket prices for the US Open have reached levels that even seasoned observers of the sport's economics are calling startling, according to a report by ONE.

Resale seats for the men's singles final on September 13 are changing hands for more than $20,900 apiece, the outlet reports, with the tournament's American Express presale window drawing enormous demand from fans chasing a seat inside Arthur Ashe Stadium for the sport's biggest American night.

It is not only the showpiece finals driving the market. A resale ticket for a Roger Federer exhibition appearance sold for nearly $12,000, ONE reports, while even first-round seats at Arthur Ashe are listed above $400. A basic grounds-pass ticket starts at $65 - the cheapest route in, though hardly cheap by the standards of American sport.

This year's tournament runs from August 31 to September 13, and the prize fund on offer has grown alongside the ticket prices: total prize money stands at $90 million, up 20 percent on last year, with the singles champions - men's and women's - each collecting $5 million.

The numbers reflect just how lucrative the US Open has become for its governing body. The United States Tennis Association reported $623.8 million in overall revenue, with $559.6 million of that - the vast majority - generated by the championship itself.

That translated into an operating profit of more than $277 million for the event, underlining why the USTA has both the appetite and the resources to keep pushing prices upward even as it invests heavily elsewhere on site.

Much of that investment is going into an $800 million redevelopment of the Flushing Meadows complex, a project ONE frames as central to understanding the tournament's pricing strategy: the USTA is leaning on record ticket revenue and hospitality demand to help fund the upgrade.

For the ordinary fan hoping to walk through the gates for a session of first-week tennis, the arithmetic is becoming steadily less forgiving - a trend ONE notes is fuelling wider debate in the tennis world about who, increasingly, a Grand Slam is actually for.