The 29-year-old, who was in the final year of his contract at the Emirates, completed his move to Old Trafford this morning for what is believed to be a £24m transfer.
"We never thought we could get Van Persie," said Ferguson. "If you go back six months ago, I couldn't see us getting him.
"I thought Arsenal are not going to let him go. When I read he had refused a new contract, that is when we acted. It has been a long haul.
"Arsene [Wenger] knew the boy wanted to leave. He knew he wanted to join us. That made it a bit easier but not in terms of trying to reduce the fee. [Wenger] could run a poker school in Govan. He got a great price but we are also happy the matter is concluded. From the starting position when we first started negotiating, Arsene has done well."
Meanwhile, Van Persie has claimed that there are no "hard feelings" between himself and Arsenal.