"The thing underlying the security" is a dynamic thing, it grows or shrinks based on the market for their products, the execution of its executives, and the quality of its products.
If it is not going to be making significantly more money next year than this year, you'd want to build that into your decision to buy the stock. Fundamental value can be defined in a lot of ways, but growth is not reliably quantifiable. Which is why there is fluctuation in stock prices.
If I believe Tesla will continue to grow at 50-100% per year, then it may be valued reasonably right now. It may even be a bargain. If, on the other hand, I believe it is a static business now and has reached saturation of its product line in the market as it exists, I'd be a fool to pay this high a price.
There are so many variables...if you're value investing rather than growth investing, TSLA is not your stock, and won't be for another decade or two. I tend to buy into growth companies, so this looks like a chance to buy into a company I missed earlier at a slightly lower price (I don't suggest trying to time the market, but sometimes the market puts something on sale that you wanted to buy anyway). It was just such a sale (only on a much larger scale) many years ago, that led to me holding a few shares of GOOG. I bought as many shares as I could afford with what was in my trading account, and I may buy as many shares of TSLA as I can afford now (after a bit of research, of course).