@dennis A few thoughts...
First, I don't like P/S as it's not very helpful to me in valuing companies. If I was to buy a company outright, I need to know their gross margins, earnings or at least earnings potential. And my offer to buy the company is going to be based on some kind of multiple of earnings or earnings potential.
Second, if you're going to use P/S then you ought to use only companies in the same auto manufacturing industry. This is only fair, since other industries have various capital costs and market forces involved. If you look at the auto industry, you'll find some of the lowest P/S ratios of any industry. It's just terrible. The reason being is because it's a very capital-intense, has high liability (ongoing lawsuits from cars built years ago), is saturated and isn't growing much as a whole.
Third, it's important to separate what TSLA fans might think TSLA is worth now and in 2020, vs what the market (made up of large investors) thinks.
Using P/S, and if you take Tesla's annual revenue run rate of let's say $10B, the market right now is giving TSLA a P/S ratio of under 3... and this is not taking into account any of Model 3. So, to say that after Model 3/Y scales to 1 million cars/year that the market is still going to give TSLA a P/S ratio of 3 is very unrealistic. The only reason TSLA is getting a P/S ratio of 3 (in an industry where most others are 0.5) is because they've got the Model 3 coming that will double or triple their revenue very soon. So, in 2020 when growth is much slower, one must reason the P/S ratio will be much lower.
Fourth, much of my reasoning for giving Tesla's auto division the valuation I'm giving is because I foresee trouble ahead for the auto industry as a whole... again, it's likely going to be a shrinking market and investors hate shrinking markets. All multiples shrink radically.
Fifth, I understand your concern that people might miss out on a huge run for TSLA. But consider the other side. Some folks are so ecstatic on TSLA that they invest at $250+ and go on margin and buy options/LEAPs, thinking TSLA is headed to the moon. And they get burnt out, and blame the shorts or the "stupid" market. The reality is, TSLA is at the price it's at now because the vast majority of investors are valuing the company like it is. I personally think once Model 3 delivers in full production, that TSLA will be a lot more solid than it is now and we'll see a decent stock appreciation. I'm looking at the stock being above $300 in 2018. But that $300+/share price is not just because of Tesla auto manufacturing... it's also because I think people are going to catch on to the value of Tesla Energy and Tesla Network somewhat.
Sixth, much of my forecasting is conservative. Further, I'm always conscious of the risk of a recession. TSLA is in a vulnerable place from now until Model 3 reaches full production. If there's a recession and let's saw the market goes down 30-35% and Tesla hits some snags, then I can see TSLA reach $100/share or lower. I rarely share this publicly on this forum because I don't want to scare people. But it's a risk that I take seriously. I maybe give it a 25% possibility (of course these odds change according to many factors).
Seventh, I think if people think the Model 3 is going to be a big success AND Tesla can succeed in one other major business (Tesla Energy, Tesla Network, Tesla Semi, etc), and that's proven correct, then they'll find that TSLA is going to appreciate a lot from now (ie., under $200).
Lastly, I wish I could go more into how I value companies and high-growth companies. I mentioned it briefly, regarding "potential earnings". But it's better explained through lots of examples and in a demo... maybe in the future, if folks are interested, I can host a google hangout and go through my method. It works well for me to evaluate if a high-growth stock is "over-valued" or "under-valued" (of course based on certain assumptions).
Oh, one more point, I do agree with those that mentioned that if Tesla blows us away with the "machine that builds the machine" and they're able to get some ridiculous gross margins on their cars, then we're going to see a much higher multiple for their valuation. For example, if they can achieve 30% margin on the Model 3... that will be hugely impressive and unexpected.