Do you realize you can buy GDDR5 modules in bulk for under $2 per 500mb module?, so you could bump a systems memory by 4gb for a BOM increase of at most, $16?
Do you think a BOM increase of $16 would increase the consoles price from the rumored $400 mark, to $1000?
That BOM is just for the chips themselves. There are additional costs.
You need a better memory controller to drive more chips, better power delivery, more traces on the motherboard... And if you need higher grade modules that clock higher or hit a lower voltage.. They come at a premium.
But you are right, it wouldn't add hundreds to the overall costs.