"The first thing we looked at was the player's ability," he revealed. "To pay that amount of money was a lot for a player in his last year. But the thing about that is that it was proved a year before when City played £24million for Samir Nasri - you either want him or you don't want him.
"If you want him, you have to pay the price the club is sticking out for. We wanted Robin badly so we were prepared to go the extra mile for him. It's just the way the game is nowadays. Obviously if you're negotiating with a club who still has the player under contract for another three or four years then they're in a better position, but at the end of the day negotiations are negotiations."
Meanwhile, Liverpool's chief threat is their top scorer, Luis Suarez, who comes into the match in fine form - even if his goal in the FA Cup at Mansfield was shrouded in controversy last Sunday.
However, Sir Alex feels there is no need to single out the Uruguayan for special attention and the normal preparation will take place for the Old Trafford encounter against the Reds' fiercest rivals.
"We want to concentrate on ourselves," explained the boss. "We pay respect to every opponent and analysis of our opponents is important. You should always give your team as much information as you can.
"Sometimes you can overdo it - going back 20 years ago we could show players a full game on video but these days that doesn't happen. They don't have the concentration.