Tonight, Elon Musk unveils the Model 3, Tesla Motors' long-awaited, affordable sedan.

Musk has already shared the car's key attributes: A 200-mile range and a $35,000 price that will drop to $27,500 or less after tax credits kick in. Tesla has long promised to change the world, and this is the car designed to bring electric vehicles to the masses and save humanity from climate catastrophe.

So the enormous hype surrounding this car is little surprise. Telsa inspires tremendous passion, so much so that people have been lining up outside Tesla stores to get their names as high as possible on the preorder list.

If you're planning to join them on that list, though, be ready to pay more than $30,000.

There are several reasons that price is almost certainly a fantasy. First, that $35,000 base model probably won't have the fun options people want. Based upon how Tesla has priced its Model S sedans and Model X SUVs, crowd-pleasing features like all-wheel drive, a larger battery for more range, and that really cool "autopilot" feature will add thousands to the sticker price. The base price of a Model S is 70 grand. Tesla won't reveal the average sale price, but Morgan Stanley pegged it at $105,000 in 2014.

But even if you're happy with the base model, there's a bigger problem. The sub-$30,000 price point is based on a $7,500 federal tax credit (some states offer additional incentives, but nothing nearly so generous). That discount is designed to foster adoption by subsidizing the cost of what remains a fairly expensive technology without dinging automakers. Everyone from General Motors to Nissan relies upon tax credits to help move cars by keeping the cost of cars like the Model 3, the Chevrolet Bolt, and the Nissan Leaf below $30,000, a number consumers find palatable.

One could argue they sort of wasted their credits on the very wealthy, people who didn't need it. Kelley Blue Book Senior Analyst Rebecca Lindland

That federal tax credit won't be around forever. It was designed to get automakers started, and applies to the first 200,000 electric vehicles a manufacturer sells in the US. Once an automaker hits that benchmark, the credit phases out incrementally over the following 12 months.

It's not known just how many cars Tesla has sold in the US since the Roadster debuted in 2008. The automaker didn't start breaking down sales by region—US, Europe, etc.—until 2014, and declined my request for that data for previous years. Still, some back-of-the-envelope math offers a glimpse.

Tesla has said it sold nearly 42,000 vehicles in the US in 2014 and 2015. Add in smaller volumes for 2012 and 2013, plus most of the 2,400 or Roadsters the company sold worldwide, and 50,000 vehicles in the US is a safe bet. That's a quarter of the cars eligible for the credit.

Looking ahead, Tesla plans to sell about 90,000 Model S and Model X cars this year. We already know that about half of all Model S sedans sold in the past two years went to American buyers, and there's no reason to believe that would change this year. With that in mind, Tesla is halfway to the 200,000 mark by the end of December.

"Ah," you say, "that leaves another 100,000 cars. No problem." Not quite. The Model 3 won't enter production until the end of 2017—and that's if Tesla meets its own deadline, something it has a habit of rarely doing. Meanwhile, it keeps selling the Model S and Model X. Even if the Model 3 rolls off the line and into driveways as scheduled, most of Tesla's federal tax breaks will be gone by then. Once they're all used up, the tax credit drops to half its value ($3,750) for six months, then a quarter ($1,875) for another six months. Then it's gone.

That could be a problem for Tesla, especially if the automaker is serious about reaching the mainstream. That $7,500 tax credit is "incredibly important," especially at the Model 3 price point, says Rebecca Lindland, a senior analyst at Kelley Blue Book. "One could argue they sort of wasted their credits on the very wealthy, people who didn't need it," she says.

For its part, Tesla doesn't seem too worried. "We build our vehicles, including Model 3, to offer compelling value without any incentives," a spokesperson said.

The fate of the federal tax credit isn't sealed, however. Lindland says Tesla could push for an extension, or other kinds of government support for buyers interested in its vehicles. "You can't let these kinds of incentives expire," she says.

Ideally, someday automakers won't need extra help to make electric cars cost competitive with their gas-powered counterparts. Lithium-ion batteries, which can make up a third of the cost of the entire car, are getting cheaper, dropping 65 percent in the past five years. Based on that trend, research firm Bloomberg New Energy Finance predicts that by 2022, "the unsubsidized total cost of ownership of [battery electric vehicles] will fall below that of an internal combustion engine vehicle.”

Until then, though, be ready to pay extra for electric.