Almost exactly a year ago, the latest market reports indicated that Manhattan and Brooklyn rental rates were on a {very} slight decline. It was the equivalent of a Mad Money "Don't Buy"-to-"Risky" upgrade, but it was still something. Since that time, Brooklyn rents have risen to their highest levels in the last five years. But something unexpected has happened in Manhattan: the rents have been on a continued decline for a few months in a row.

"All of sudden, in late September, the market just died," Marc Lewis, the managing chairman of rental brokerage Coldwell Banker AC Lawrence, told the Times. “There’s been a strong drop-off in demand, and landlords have lost a few months’ rent." While it's normal for there the market to slow down in winter, this has been a steeper and longer decline than in recent years.

Take a typical one-bedroom walk-up, for example. In September, it rented for an average of $2,512 a month, the firm’s data show, but in October it fell to $2,326, or more than 7 percent. In November there was a further decline of nearly 2 percent, to $2,282, and in December yet another 2 percent slide, to $2,238. Last month, average rents slipped again, according to other firms’ data.

Landlords have noticed the change, and are acting accordingly: the Times says about 30 percent of rentals were offering some types of concessions (such as a month of free rent, landlord-paid broker fees, or both) as of January, the highest rate since 2010 (some other reports put that figure at closer to 13%, still an increase from where it was six months ago). This has helped bring down vacancy levels, which went from 1.82% in December to 1.62% in January.

It's not all wine and roses though: as the Journal points out, the average Manhattan apartment rented for $3,397 in January, $186 more than in January 2013 (the News puts the median rent at $3,114; average rental prices tend to be higher than median ones). That's because up until the drop-off in the fall, rents were moving ever upward for the first half of 2013.

Here's the most remarkable bit of data: the price gap between Manhattan and Brooklyn is now just $284, the smallest difference in at least five years. For some context: the gap was closer to $1000 back in 2008. But of course, that was before all the aliens and Girls fanatics moved to town.

Suffice to say, you'd probably still be better off buying a small island if you have the cash.