Is 10% a reasonable value for that chance? Is 1% a reasonable value for the ownership stake? What about dilution and payout rights?
A reasonable expected value of a lottery ticket is directly computable: jackpot times probability of a winning set. I'm not so sure the same is true for the value of startup employment.
Lottery tickets' jackpot is usually determined in part by sales of tickets for that drawing, and a winning combination wins a divided pot in the case of multiple winners (the probability of which depends again on sales of tickets for that drawing.)
So, no, you can't, at the time of buying a ticket, actually calculate more than a guess of they parameters you cite for determining its expected value.
The published jackpot is a very accurate value to use, and the probability of being the sole winner is a very reasonable probability to use. The actual expected value is not going to differ from that product by enough to matter most of the time.
A reasonable expected value of a lottery ticket is directly computable: jackpot times probability of a winning set. I'm not so sure the same is true for the value of startup employment.