You can find cheap used enterprise SSDs on ebay. But the problem is that even the most power efficient enterprise SSD (SATA) idle at like 1w. And given the smaller capacities, you need many more to match a hard drive. In the end HDD might actually consume less power than an all flash array + controllers if you need a large capacity.
Agree on SSD for cold storage, that's not a good idea. But you would be surprised by how little used are typical used enterprise SSDs on ebay. This article matches my experience:
https://www.servethehome.com/we-bought-1347-used-data-center...
I bought over 200 over the last year, and the average wear level was 96%, and 95% had a wear above 88%.
Endurance and retention are inversely correlated, and as I mentioned in my original comment, enterprise DC drives are designed to advertise the former at the expense of the latter. The industry standard used to be 5 years retention for consumer and 3 months for enterprise, after reaching the specified TBW. The wear level SMART counter reflects that; "96% remaining" on an enterprise drive may be 40% or less on a consumer one having written the same amount, since the latter is specified to hold the data for longer once its rating has been reached.
Retention is offline retention. Not online. So not sure what point you are trying to make. If it is that SSDs shouldn't be used for cold storage, yeah I agree, and enterprise SSds aren't designed for cold storage. But you seem to be linking retention to TBW, which are largely orthogonal metrics. If you are going to use the SSDs in a NAS, which by definition are running all the time, why would you even care about the rentention rating?
Used SSDs, especially enterprise ones, are a really bad idea unless you get some really old SLC parts. Flash wears out in a very obvious way that HDDs don't, and keep in mind that enterprise-rated SSDs are deliberately rated to sacrifice retention for endurance.