"Jermain, again as I've said, it's the whole package .... to pay a transfer fee and then pay the wages and everything would be too expensive for us at the moment," Redknapp said Wednesday at a news conference before Thursday's game against Swansea. "So that is the problem, the package wouldn't be within our budget.
"I like Jermain, he's an excellent player, a great lad, but it'd be a difficult one."
Defoe's future with Toronto FC has been up in the air since the MLS club turned down a franchise-record transfer bid at the end of the summer transfer window for the 32-year-England star.
Redknapp says he is in the market for a striker to help Charlie Austin when the January transfer window opens.
"You can't rely on Charlie getting all the goals, that's obvious," he said. "So that is an area we're looking at. But it's very difficult. When you're looking for loans, who's going to loan you a striker who's going to come in and go and grab you 10, 12 goals between now and the end of the season? ... It's just so difficult finding strikers. Everybody's looking for a striker."