One other factor you should take into account:
With the current market surrounding memory and manufacturers shifting their focus from the consumer market to the B2B/cloud computing market, prices on newer phones – especially those not on the same level as Samsung/Apple – might increase in the coming year(s).
If the specific manufacturer GrapheneOS is working with, has locked in memory prices on a long-term contract, this might not affect them as much as others. Seeing as the phone is in active development, this might very well be the case.
It was mentioned that the OEM working with GrapheneOS is one of the "top 10" but depending on their market capitalization (i.e. lower half of the the top 10), their long term memory price stability might not be on the same level as giants like Apple and Samsung.
Of course, this is all speculation and I would like to reiterate that I am not an expert and am dealing with second-hand information which might not be accurate and might not properly portray the facts as they are.