Well if you bought a 0.001% stake for $1000, then the valuation would be $100M, not $1B. You probably mean either $10000 or 0.0001%. Regardless, that's the idea, yes. If a company had a million shares, and you bought one for $1000, you're essentially saying that the present value each of those shares is worth $1000, so the company is worth $1B.