Modric has expressed his determination to secure a move to Stamford Bridge and Redknapp, who reiterated this week that he "understands" the player's desire to leave, has claimed many of the world's top clubs are interested.
On Thursday, he said: "Everyone wants Luka. Just ask Alex Ferguson what he thinks of him, ask Roberto Mancini at Manchester City - they all want him. He could play for Barcelona."
However, Chelsea - who have already seen offers of £22 million and £27 million rejected - have been told that they will need to dramatically up their bid.
Asked whether £35 million would be enough at a press conference on Friday, Redknapp said: "If we were looking at selling, it wouldn't be my valuation.
"I think he is worth an awful lot more money than that, but we are not looking to sell Luka. We need to keep our best players, so he is a player we want to keep if possible.
"It's in the hands of the chairman of the football club - it's not my decision. I work with him as the coach, but the chairman says he is not for sale so we will have to wait and see. He is quite determined, if we can, to hang onto him.
"It's a difficult situation. It's a difficult one for Luka and it's a difficult one for Tottenham."
Chelsea boss Andre Villas-Boas, meanwhile, says he wants to take his time before making major moves in the market.
"Everyone knows how frenetic the market gets in the last weeks of August," he said. "It always happens like that and, although I understand the fans want and expect new signings and radical changes, we are trying to put a halt to that because it is valuable that we are assessing things.
"Daniel Sturridge, Ryan Bertrand and Patrick van Aanholt are back from loans and this has a value for us and it must have a value for the Chelsea supporters as well, because we are not making decisions lightly.
"We are pondering every situation in order to go to the market with the things that we really need and not to go into the market chaotically and make mistakes."