This is a real hard and worse - an emotion ridden subjective question. After all "how dare you demand 'more' when the founders ate ramen?" Etc.
Couple of factors I'd consider (and despite that still made mistakes):
Assumptions:
* This is in bay area.
* You have 5-10 years of experience
* You have a competing FAANG offers or can get one with 2-3 months of leetcode grinding
Option A - FAANG job
* Assume TC of 300k a year (conservatively). Includes base, bonus and real equity.
Option B - The startup:
* Pays say 200k in base salary
* assume aggressive valuation of 50M making .2% = 100k over four years
Say your exit is in a 8 year time frame. (Startups going from 10 employees to an exit faster than 8 years is very unlikely??).
Option A (assume no pay increase) over 8 years:
2.4M
Option B with a 100x growth in 8 years:
200 x 8 + 100k x 100 = 1.6 + 10M = 11.6M
Now do you believe their "story" on why they think they are gonna grow 100x in 8 years (without you incurring dilution etc)?
Do you believe you can get to 500k "real" tc at another FAANG in 3 years (say by moving around)?
Which scenario seems more likely and safer?
If you dont think 100x is possible and 50x is more realistic and you still need get your 11.6M to make it worthwhile (not making much real moolah for 10 years) then perhaps 0.4% would break even?
Now there are plenty of unicorns which would do >>>> than 100x. If only I could pick them before the fact!