Dubai-based investment holding company Istithmar has reached an agreement to sell the Helmsley Building at 230 Park Ave. back to Anthony Westreich of Monday Properties and Goldman Sachs for $1.15 billion, The Post has learned.

The 1.22 million-square foot building, which straddles Park Avenue between 45th and 46th streets, was purchased from investor Robert Bass at the end of 2005 for $705 million.

“That’s not a bad day’s pay – $400 million for about 18 months of ownership,” said one real estate executive familiar with the deal. At $1,000 per foot, the sale is in line with other Midtown sales.

Neither Farkas nor Westreich returned calls for comment.

Because office buildings are so desirable and replacement costs are so high, the executive noted that they could easily spend $1,000 per foot on just the land alone.

Monday Properties had previously invested in the building and at one time handled managing and leasing for the building when its predecessor company purchased it from Leona Helmsley.

Andrew Farkas acted as broker on the sale and had advised Istithmar on its original purchase.