After seven years in her $2,280-a-month co-op apartment at Sutton Gardens on the Upper East Side, Gail Bianco was surprised this September when her landlords gave her a mere three weeks notice that they would not be renewing her lease.

But that was nothing compared to the shock she got when she asked for more time to clear out.

Five days later, she and her husband were given a sublease agreement demanding $3,000 rent for the month of October and an additional $3,000 security deposit. And for every day they stayed past the end of October, they would be charged $500 a day–a total of $15,000 for the month of November.

“I was stunned,” says Bianco, who contends that she never missed a rent payment, a claim her building manager contests. “We had expected to be there for many, many years. I was upset and angry that they were giving us such short notice. We didn’t deserve this kind of treatment. We’d been there for seven years. It was more than unethical. It was nasty.”

It may have been nasty, but it probably wasn’t illegal. The Biancos are now locked in a court battle over September’s rent, but the landlords, Sutton Associates, say it’s completely within their discretion to refuse to renew the lease and charge any rent they want, since the apartment’s not rent-stabilized.

“When [the landlords] hadn’t heard from her at the end of August, the decision was made to put the apartment on the market,” explains Sutton Gardens managing agent Mike Noble, who claims the Biancos got renewal notices in both April and August.

Questioned about the $500-a-day offer, Noble justified the rate: “That’s what it would take to rent a hotel space that size”–approximately 1,100 square feet. “This is not some little old lady living hand to mouth,” he continues. “These are people who paid $2,280 a month for an apartment.”

Angelita Anderson, executive director of the City-wide Task Force on Housing Court, says more co-op renters are getting kicked out as the sales market heats up. “We’ve seen this practice, where people have to pay these enormous sums,” she says, “but this is the most outrageous sum I have ever seen.”