It's hard for me to tell how much RAM upgrades are worth on an SoC, I don't have any reference.
It's not hard - Apple have been charging the same $200 for an 8 to 16GB RAM upgrade since
at least 2017 when iMac used bog standard DDR4 SODIMMs. In 2017, I paid £140 to
add 2x8GB of 3rd party RAM (the self-same brand of Micron sticks that Apple used) c.f. the £200* that Apple wanted to supply 2x8GB
instead of 2x4GB (at retail that would have been an added cost of about £70). Obviously those 3rd party prices are
retail so they included a profit margin for the distributor.
(* UK tech prices tend - to a good first approximation - towards the US price with the $ sign replaced by a £, regardless of exchange rates)
Same goes for SSD - difference between the (then optional) 256GB SSD and 512GB has been $200 since at least 2017.
Essentially, these are strategic price points chosen by Apple based on what they think the market will bear and have very little to do with the actual bill-of-materials. If it was actually cost-plus-reasonable-markup driven I doubt that the extra logistics costs of making and distributing 8/256 models would be viable.
The fact that base RAM of the iMac has been 8GB
since 2014 should be criticism enough. Apple are really pushing how long they can get away without improving their base specs and charging so much for upgrades.