You can't prove it, because it's not true. I've been using/purchasing hard drives for nearly 20 years, and I KNOW the cost has come down. I know, because I was *there*. 20 megabytes was $300 in the late 80's. Today you can purchase an ultra- tiny drive about $50. The price came down.
In the late 80's builiding a hard drive-equipped console like X-box would have been extremely expensive (~$1000). Now, we can do it for ~$300. Why? Because PRICES DROPPED on hard drive manufacturing costs.
Anyway to me, your assertion is as silly as saying, "There's no difference between 1980's and 2000's cars." It's just a stupid remark.
(Note I said your remark was stupid, not you. I criticize ideas, not people.)