I haven't seen people talk about it, really, but the White House has a maintenance budget of between $10 and $20 million a year as it is now. This new building has almost double the footprint of the entire existing structure.
Even if he were paying for it with his own money (which of course he isn't, because it's 1) not his and 2) the whole "the Justice Department owes me $230 million" didn't come from nowhere), or with corporate bribes, taxpayers are still going to be footing the bill to maintain it.
Considering the size of the thing, the security costs, and the costs to staff it, it's reasonable to assume that this new building will cost taxpayers at the very least an amount comparable to the current White House budget to maintain. That means taxpayers are going to have paid as much for this building as these "investors" by the time twenty years pass. If it stands as long as the East Wing did, ignoring inevitable inflationary costs and assuming a conservative estimate of ~75% of the current budget annually, it'll cost taxpayers close to $1.25 billion over its lifetime. It's not unreasonable to estimate a total cost around twice that amount.
The bottom line is that, even if Trump pays for the construction with private money, the annual cost of maintenance for Trump's ballroom is likely going to be roughly around half what a state the size of Montana is
losing in SNAP benefits annually as a result of the Republican reconciliation bill this year.