When I last looked into this, flying private started somewhere around 10k a seat. Not cheap, but also something non-centimillionaires can afford.
Looking it up again, it looks like the way they generally charge is per hour of flying. Low end is $2k per hour with the high end being $10k per hour. I'm guessing the type of jet and accommodations makes all the difference.
Then "true cost" requires a rabbit hole of business expense write-offs, fixed v. marginal costs, speed, comfort, (sub-)leasing, which airport you fly into, O&M strategies, and more.
Per the article, your fractional share of a 10M plane with 1M/y of maintenance gets you 100 hr/y.