Prieto hit his third with 14 minutes to go but Sociedad also finished with 10 men as Daniel Estrada was given a second yellow card.
After the game Mourinho said he was unconcerned about receiving abuse from the crowd.
"Perfect. Let them whistle me before the match and then support the team as they are doing. I think the whistles are for leaving Casillas out and also because we are not achieving our aims in the championship," he said.
"There are others who didn't applaud and are not happy with his (Casillas') performances. If I am not doing well it is normal that they whistle and it is something you have to live with. I don't want to be a hero independently of my performances."
Madrid faced an injury crisis at the back with Pepe, Fabio Coentrao, Marcelo and Raul Albiol all injured and Sergio Ramos suspended.
After the break the game hit a lull before Ronaldo responded with a double.
Benzema hit a ball over the top and he finished clinically and then a couple of minutes later he struck a free-kick into the top corner.
At the Camp Nou, Pedro Rodriguez hit a double with Xavi Hernandez and Lionel Messi also scoring in a rampant first half as Barca welcomed coach Tito Vilanova back after cancer surgery.
"I would like to thank everyone for their support. This club has had a lot of knocks in the last couple of years but I always want to take the positive approach. What has happened to me could have happened to anyone," said Vilanova.
"This is not over yet and there will be days that I can take training and others I can't but I want the media to realise that this illness is mine and it is private."
Vilanova chose to play Andres Iniesta in his three-man attack and leave Alexis Sanchez and David Villa on the bench against Espanyol who were unbeaten in four games under new coach Javier Aguirre.
Iniesta crossed for an unmarked Xavi to put Barca ahead after 10 minutes and Cesc Fabregas pulled the ball back for Pedro to score from close range.
Pedro produced a neat finish to beat keeper Francisco Casilla midway through the first half and then Messi struck from the penalty spot after Fabregas was felled by Casilla.
-AAP