My computer had 16Mb in 1997, and it was lower-range but not the absolute bottom.

It looks like Anandtech listed 128Mb for $300 (not inflation adjusted) in 1997. It fell to $150 in 1998 and by 1999 you could buy it for $100.

So 512Mb RAM by the end of 1999 for ~$200 was plausible.