It's not a good guess, but it's one of the best guesses we have.
The other guess is speculation based on the investor's input into the Zcash company. Investors have put in $3 million total. Investors will receive 3.3% of the first 4 years of Zcash, and 1.65% overall. This puts a valuation of the Zcash company at $32 million (Investors own 16.4% of the Zcash company).
Math time. Investors get 3.3% of the first 10.5 million ZCash. 0.033 * 10,500,000 = 346,500 ZCash rewarded to investors.
They paid $3,000,000 / 346,500 = $8.65 / Zcash, approximately 10% of the 'Futures' price table posted above.
This is only one of many ways to calculate what investor's paid. You could do it based off of the 2nd investor's input of 2 million, or the first investor's input of 1 million, or you do an average which I did above.
There is a lot of clarification and also corrections ITT: https://forum.zcashcommunity.com/t/founders-should-only-receive-10-of-the-first-halving-and-beyond/1837