But since they're using similar APUs and since Sony has a less complicated version of the same APU, it will still benefit Sony greatly too
Not really. MS went with cheap ddr 3 ram and a larger die for edram. As the die size decreases with each micron drop they are able to fit more chips on a wafer .
While sony's die will also get smaller their ram GDDR wont go down in price as quickly since its mostly used in video cards and the ps4 itself will be the main driver of cost reduction. Lets also not forget that they are using a cutting edge density.
Sony is most likely going to be stuck with the same amount of gddr chips for the majority of this generation.