Sakho had a big impact on loan at Selhurst Park last season, making eight Premier League appearances - including five wins - as Palace preserved their top-flight status.
But the France international has since returned to Liverpool, who are reportedly asking for £30 million from Palace to make the move permanent - a fee that De Boer is unable to pay.
"Everybody knows [Sakho] had a major impact last season and, when he's available for us [to buy], I think that is a quality injection for the club," the Palace manager said.
"It is not like this now because he is much too expensive for the club."
Steve Parish, the club's chairman, added that he has been stunned by the figures he has seen other clubs spend in the transfer market.
"What's returned to football are the big-money owners, the win‑at‑all‑costs owners, almost bottomless pockets," he said.
"I'd love to see how it all fits with Financial Fair Play, by the way, because I just can't see how half of this fits with that because the revenue of the clubs hasn't grown this year.
"I'm not accusing people of anything. I just don't understand the numbers - for some of the clubs - that they're spending, that are being quoted and how you can make that fit over a period with Financial Fair Play."
Palace have lost their opening two games of the season, to Huddersfield Town and Liverpool, without scoring.