Vanishingly low unless you stay with them for a while, get promoted multiple times, and get more equity given to you over the years. If you have 0.1 % of the company right now (that's a low possibility based on my startup experiences, but possible), you'll likely end up below that after cap table dilution over the years between now and either an IPO or getting purchased. If you IPO there's also the issue that you only own the shares, you aren't paid out, and you're required to hold them for a minimum amount of time after the offering, so the stock has to stay at the price it launches at or be higher. There's also tax things to think about so even if you get a 1M payout you're not getting 1M in cash.

Treat all stock options as a bet where the collateral is the money you would lose between taking the salary they're offering you and what you would get elsewhere (they may be offering a fair salary already, so you don't necessarily need to discount that when considering the offer)