The 29 year old has scored 20 goals and a growing number of assists this term as United hold a seven point lead at the top of the Premiership.
"The first thing we look at is the player's ability," said Ferguson. "It was a lot of money for a player in the last year of his contract.
"You have to pay the price that the club are sticking out for, whether it is one year on his contract, two or three.
"We wanted him badly, so we were prepared to go the extra mile.
"Any player coming to us for the first year, you never know for sure. Who would have thought Wayne Rooney would have got a hat-trick on his debut? It is fairytale stuff.
"But what we did know was that we were signing a very good player."