There have been many things working against the Nets' proposed arena in Brooklyn and the cost of protecting it against terrorism might be the final element to end the project.

The bulletproof glass facade proposed for the glitzy arena will cost a mind-blowing $625 per square foot, a source familiar with the designs told The News.

"I think the owners clearly didn't have their financing tied down for this project, and that's going to be the biggest hurdle," said the source, who spoke on condition of anonymity about the sky-high prices associated with securing the 850,000-square-foot arena against terrorism.

"With the security concerns at the arena, there's not much you can do to make it that much cheaper," added the source.