While on paper they say this grant could be worth $1m if startup is valued at $1b, it's still only 0.1% of the company and $50k at the last valuation.
Strike price is really low but I am still wondering if I am being lowballed?
Also don't buy BS like "they say this grant could be worth $1m if startup is valued at $1b". Not only is there a small chance of them ever getting to a $1B valuation, you would mostly likely be diluted between now and then by additional funding rounds.
If you believe management is amendable and just needs data, then pull data for typical equity grants at this stage to your equivalent position and present it to them.
If not amenable, then decide if you are prepared to walk away and then if so set an ultimatum for a market-rate grant.
Also, regarding "$1m if startup is valued at $1b, it's still only 0.1%" it's actually quite likely to get diluted by that point. You should ask them if the shares are non-dilutive or not (unlikely to be protected from dilution).
Were you not given equity when you started a year ago? Was it still Series A at that time?