The government predicted 3000 children would leave/not enter the private school sector in the 24/25 school year, rising to 14k next year (3%) and eventually 37k (7.5%)
Whether that is correct makes a huge difference to the eventual revenue/cost because when a child is forced out of private, the exchequer not only loses their predicted VAT income, but also has to start paying £8k per year for them: like they do for every other UK child.
More if the child has SEN - which is disproportionately the case for those having to leave, since parents whose children aren't coping in state may push themselves to the limit to pay fees.
There's also a loss of school employment when they have to cut back/close: not only for teachers but for cleaners, groundsmen, caterers etc. The government predicted 100 schools would close eventually: there have already been more than 50 in the first year.
The accepted numbers are:
0% leave - revenue of £1.67bn
5% leave (25k children) - revenue of £0.84 bn
10% leave (50k children) - no financial gain or loss, only the impact on those kids
15% leave (75k children) - £0.83 bn loss
20% leave (100k children) - £1.67 bn loss
25% leave (125k children) - £2.51 bn loss
Note that this number includes children who never start private school - but would have without the tax - as well as those who leave.
Actual behaviour is starting to trickle through now, although it will be a few years before it becomes clear because parents avoid removing children partway through their education.
13K left throughout academic year 24/5 - more than 4 times the government prediction of 3K.
If that continues, they're into break-even at best, and likely a loss - perhaps up to £2.5billion per year loss eventually. Whilst of course her 'fully costed budget' factored in the £1.6 billion she pretended it would raise. Quite the addition to Rachel's black hole. No wonder they're refusing to answer.