Mayor Bloomberg defended the city’s offer to purchase the Hudson Rail Yards for $500 million, saying the site has been an “open sore” and that the city has a vested interest in what is developed there.

While some have criticized the city’s offer as too low and say the undeveloped land on Manhattan’s West Side is valued at $900 million at the very least, Mr. Bloomberg called the city’s offer a “fair price” and argued that the city has no interest in shortchanging the Metropolitan Transit Authority, the state agency that owns the site.

“The city owns our transit system. We lease it to the MTA,” the mayor told reporters yesterday.

On Friday Mr. Bloomberg and the speaker of the City Council, Christine Quinn, wrote a letter to the head of the MTA, Peter Kalikow, asking that he consider the offer at this month’s board meeting. The mayor said the city should have a say in how it’s developed because whatever ends up there will shape the area for decades.

Yesterday, Mr. Bloomberg denied that his administration was trying to rush approval of the sale through the MTA board.

“This is nothing that’s happening overnight. This is something that’s been happening for a very long time and I’d be derelict in my duty if we didn’t try to get this going as fast as possible,” Mr. Bloomberg said.

The site is the same place that Mr. Bloomberg wanted the Jets to build a new football stadium last year. That plan failed.