California’s latest proposed tax increase has already led to so much wealth fleeing the state that tax revenue is set to fall, not rise. Golden State politicians announced the upcoming opening day of hunting season on billionaires and are about to find out they can’t fund welfare programs by taxing people who have already left.

At issue is California’s Proposition 40, a one-time 5% tax on billionaires’ net worth, which proponents claim will raise $100 billion for state-funded health care. Unfortunately for them, the wealthy were paying attention. At least seven high-profile billionaires left the state before Jan. 1 of this year, the residency date that determines who owes the tax.

They took with them roughly $539 billion of the state’s $1.9 trillion in billionaire wealth. Google co-founders Larry Page and Sergey Brin alone shrank the expected haul by $25 billion.

The fact is, $100 billion was never a realistic revenue projection. That estimate ignored losses from departures and legal challenges and both are certain to keep coming. The retroactive residency date alone invites a raft of constitutional challenges.

Supreme Court precedent bars retroactively imposing a “wholly new tax.” Locking people into paying whether they stay or leave, before voters have even approved the tax, burdens the constitutional right to travel. And as the Tax Foundation notes, uncapped penalties on appraisers of hard-to-value assets raise Excessive Fines Clause concerns, since they’re grossly disproportionate to good-faith errors.

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Once departures and litigation are counted, Stanford economists Joshua Rauh and Benjamin Jaros estimate the tax would bring in closer to $30 billion. That’s less than one-third of what was promised, which is bad news for Medi-Cal, the state’s Medicaid program.

But the bigger problem has already arrived. While a wealth tax is collected once, an income tax is collected every year. Rauh and Jaros estimate California’s billionaires pay between $3.3 billion and $5.8 billion in state income tax annually, and every billionaire who relocates takes his share with him permanently.

Weigh that lost income tax revenue against Prop 40’s one-time haul, and the net present value of the tax comes out to negative $38.9 billion.

In other words, the tax is a money-loser for the state. And this estimate isn’t a mere outlier from unlikely assumptions: economists ran 100,000 simulations with different revenue estimates and discount rates, and the state came out behind in 85% of them.

California already depends on a sliver of total earners for most of its tax receipts, with the top 1 percent of earners supplying roughly half of all personal income tax revenue. These are the people creating business, funding startups, and employing thousands of Californians. Losing even a handful of them does real damage to state and family finances alike.

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Ironically, the billionaires who flee suffer less than the average Californian. As F. Scott Fitzgerald said, the rich are different from you and me; they can leave for Florida, Tennessee, Texas, or another low-tax state. It’s California’s public services, funded by their income, that will suffer.

Medi-Cal and the other programs counting on this money will still need funding. When $100 billion becomes $30 billion, followed by a permanent reduction to income tax revenue, politicians will clamor for more tax hikes.

But with a shrinking tax base of billionaires, the hikes will inevitably fall on the middle class. A tax sold as making billionaires “pay their fair share” will end up costing the residents not wealthy enough to leave.

Supporters will point to research showing high-earners rarely move because of taxes. But the best-known study examined ordinary income tax rates over many years. A levy on someone’s entire net worth is a different animal and will prompt more people to leave, as seven just did.

Before Californians vote on Prop 40, they should understand how much the tax will realistically raise—and who will get stuck with the actual bill in the long run.