The 27-year-old Frenchman, who made eight appearances on loan for Palace last season, is behind Dejan Lovren, Joel Matip and Ragnar Klavan in the Reds' centre-back pecking order.
It was reported earlier this summer that Liverpool would let Sakho go if their £30m valuation of the player was met, but De Boer says he is too expensive for the Eagles.
"Everybody knows he had a major impact last season and, when he's available for us, I think that is a quality injection for the club. It is not like this now because he is much too expensive for the club," he told The Guardian.
Liverpool signed Sakho from Paris Saint-Germain in September 2013 for £18m.