Anecdotal, you're quoting one site with individual prices for individual modules. You can just as easily find 256Mb PC133 for ~$15-20. You're also selecting a very specific chip from each generation. Why don't you choose a 512Mb PC3200 (even from Crucial Direct) to represent the common 2005-era module? Would it be because its just $43, which is even less than half of the 1Gb DDR2 module you priced. Heck, why not choose better top end modern-era RAM at $200?
You're clearly failing to grasp terms like "trend" and "average" and instead interpreting them as "literal price according to this price guide right here!!!11!!!!111 shift one"
Of course the price doesn't literally convert to half right down to the penny! But the overall price of all the circuit-based components is halved as an average of each ones' individual rate of depreciation, which has surprisingly been steady at ~50% every two years for over four decades.
