The 25-year-old has netted seven already in his first season in the Premier League and with just 18 months left to run on his current contract, Redknapp is keen to keep him at Loftus Road.
"Do we need to stay up to keep him? Staying up will be very important," Redknapp said. "He's happy here and he wants to play in the Premier League now.
"He's had a taste of it and once you play in the Premier League you want to stay there. If he can go and get 20 goals this year that will help us do that."
Austin moved to Rangers from Burnley for £4m in August 2013 and goes up against another of last season's Championship heroes, Clarets forward Danny Ings, when the sides meet on Saturday.
"How much is he worth now? I couldn't put a price on him, he'll start wanting a new contract. I don't know what he's worth, but certainly more than £4m," added Redknapp.
"They both scored goals last year in the Championship.
"Ings is in the England U21 team, I certainly expected him to score goals this year and Charlie has been the same, he's carried it on.
"If they were playing in teams at the top of the league who were making dozens of chances every game they would get a lot more goals as well.
"It's harder when you're playing for a team lower down, you don't get so many chances and you have to take those that come along. That's what they're doing at the moment."