Remy has scored 10 goals in 19 games for the Magpies since joining the Barclays Premier League club in August on a season-long loan deal.
The French forward is due to return to Loftus Road at the end of the campaign, with more than three years still to run on his deal with the Londoners.
Newcastle manager Alan Pardew is keen to keep the 27-year-old on Tyneside, but Redknapp has warned he will not be sold on the cheap.
"You would think they would want to snap him up because if they don't somebody else will for that money," Redknapp told the London Evening Standard.
"We paid £8million and he's worth double that because of the goals he's scored in the Premier League.
"He would be a great player if he came back, but he wanted to go. He's not going to want to come back here and play in the Championship.
"If someone came in and offered £16million for him I would sell him if it was my choice."