The only evidence for that btw is... Donald trump says so. It's in the plan for it.
2010 Private Citizen trump approached David Axelrod of the OBama administration and said you have these tents for state dinners, it's tacky, I am, a builder I build ballrooms the greatest ballrooms anybody has ever seen come to Mar A Lago and see. Aexlrod dumped it on someone else in the administration who didn't follow it up as spending 100K on it as proposed by Trump was a waste of taxpayer money. Trump fumed told friends he was ignored and has wanted to build it ever since
But yeah NOW it's going to be privately funded.
Just like the jet is "free."
Or that secret service staying at Trump properties saves us money