Earlier quoted context omitted.
There is no "objective way" when it comes markets, there is only what the market will give you. If the market will give you $450M for 5% of a product that doesn't work (a la Theranos), then objectively you're worth $9B, at least until you go bankrupt and then you're worth nothing. If the market is dead but you somehow manage to IPO anyway, save the company, and sell for $1.5B, you're worth $1.5B (a la LoudCloud/Opswa…
So technically you can raise $1M for 0.1% of your startup and give the VC some crazy 20x liquidation preference. Now the market has you at $1B in valuation.
http://blog.samaltman.com/the-tech-bust-of-2015
I could drop the market cap of Google down to $3.4M right now. All I have to do is sell one of my shares for $0.01. The thing is, it would pop right back up again to $483B within a few milliseconds, and I'd just be out $690, so there's kinda no point to it.
(Pedantic note, since I know there's gonna be someone in the financial industry that corrects me: no, I couldn't, technically. When I put in a sell order, it goes into the order book, and buyers are required by law to take the best offer, which is probably more than mine. I'd have to place my order at a time when there are no outstanding limit offers. This has actually happened during flash crashes and technical glitches, but is not a normal occurrence.)
Privately traded companies are similar, but because there's less liquidity, the price doesn't necessarily correct on any reasonably time scale.