By nearly every valuation metric possible, Tesla Motors (NASDAQ:TSLA) is grossly overvalued. Traditional valuation metrics are basically rendered useless in analyzing Tesla. The company's iconic CEO, massive market opportunity, and unreal execution give the stock rights to an intangible premium that is basically impossible to quantify. Like it or not, however, this vague, intangible premium is real. How in the world, therefore, can an investor decide at what price Tesla is a buy?

But why?
Investors can't deny the fact that certain companies simply persist in defying conventional valuations for decades (think Amazon). It makes sense that companies with genius-like management and market opportunities that seem to have no end would deserve an enormous premium. But where does this premium come from? Why does disruption deserve an intangible premium?

The answer isn't mysterious. It comes from confidence. Genius talent, exceptional market opportunity, and a record of bulletproof execution provide investors confidence in the likelihood of the best-case scenario. We could go even further: Factors like these provide confidence that the company can outperform the best-case scenario.

In 2001, very few thought Amazon would challenge Wal-Mart. Apple beat best-case scenarios time and time again, revolutionizing entire industries with the iPod and the iPhone, and challenging PCs with an entirely new category: tablets. Amazon and Apple didn't just match their best-case scenarios -- they accomplished the impossible.

So, how big of a premium does Tesla deserve?
Led by the indispensable Elon Musk, the company has eloquently positioned itself as the top-notch, purely electric brand of the future. Even more, it has arguably hedged its own success with its aggressive expansion of exclusive, superior charging infrastructure. This has given investors an unquantifiable confidence in the company's future. In fact, investors are so confident in the company that the stock is priced for successful mass production and adoption of its third-generation affordable car.

Tesla's current premium is unquestionably large -- maybe even overly euphoric. If Tesla meets the goals it has laid out for next year, the company could generate $1.2 billion in gross profit for the full year of 2014. That's based on non-GAAP gross profit excluding benefits from ZEV credits, assuming Tesla exceeds 40,000 vehicle sales in 2014 and maintains its fourth-quarter gross profit margin target of 25%. In other words, Tesla may be trading at 17.6 times an optimistic outlook for next year's gross profit. Comparatively, Ford and General Motors trade at about 2.9 and 4.7 times a reasonable guess for their 2014 gross profit, respectively.

So, back to the question: What kind of a premium does Tesla deserve? I'm not going to pretend to have an exact number for you. What I can say is this: It's enormous. But exactly how big? Who knows? Personally, 17.6 times an optimistic outlook for next year's gross profit left me too uncomfortable to continue holding, leading me to sell shares last month. If the stock sells off significantly, I may be interested again. Sure, that may never happen. But that's OK. Jumping in on so much uncertainty is not investing -- it's gambling.

This vagueness is the unfortunate nature of investing in disruptive companies with enormous growth prospects already priced into the stock -- there's a lot of uncertainty. But don't throw all reason aside and invest in the stock at any price just because conventional valuation metrics don't apply. Nor should you react in the opposite direction and never invest in game-changing companies. Instead, approach each disrupter carefully and decide on a premium you feel comfortable with. Do your research and use discretion.

Most of all, don't be afraid to let investing be an art and not a science. As long as you're buying great businesses for reasonable prices with the intention of holding for the long haul, the science of investing seems to take care of itself. The key term to be emphasized in the former sentence for Tesla investors is reasonable prices.

Fool contributor Daniel Sparks owns shares of Apple. The Motley Fool recommends and owns shares of, Apple, Ford, and Tesla Motors. It also owns shares of General Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.