Van Persie scored a sublime equaliser for Saturday's FA Cup draw on Saturday.
It was the striker's 20th goal on his 26th appearance for Sir Alex Ferguson's giants.
Asked if van Persie's price was looking like a bargain, West Ham boss Allardyce said: "Yes, isn't it?
"The man himself picked the team where he thinks he can get the most trophies, not make the most money, and that is to his credit.
"Because he is at that team he wants to be successful, and as a professional he wants to hit the ground running. He is already shown everyone what he can do."