200km fiber can hold working memory of 32gb. Market prices 200km single mode fiber at ~$320, slightly cheaper than DDR5, but uses way less power.

Source: https://twitter.com/ID_AA_Carmack/status/2019839335382790342 (https://news.ycombinator.com/item?id=46916548)

Time domain mass storage is a hell of an idea. Average access time scales inversely with the number of loops used. Fiber is really cheap compared to silicon or copper. I bet you could make a special form of it just for this application that would be even cheaper.