its really hard to evaluate stock option.
You can roughly estimate their monetary value if you can get the following
information.
In their last round of funding, how much money they raised, and what
percentage of company did the investors get. For example, they raise 30
million in series B, and the investors get 10% of the company, the company's
valuation is at 300 million.
Then, the company is willing to tell you how many outstanding shares are out
there, you can calculate how much money your shares are worth.
However, some of the information is not publicly available, and most
startups are not willing to tell you how many outstanding shares are out
there. Even if they do tell you, and they can easily increase number of
shares, and in return, diluting your value.