TBH I'd use my HDD docks and just place my unused HDD in there if I were to run a full node. Pretty sure people with the knowledge on what to do with Raspberry Pi and how it works would have had some extra PC components lying around and I'm one of them (lol). Anyhow, knowing that $/GB has been pretty much going down year by year, it's easy to find the right storage device if one is to run a node. Everything is coming cheaper for the PC industry these days, well at least the outdated (?) components.
It's not like spare parts are worthless, you could always sell your old HDD and dock to a guy that wants to run a bitcoins node for 25$!
So, it's still a small investment on top of those 35$.
And I wouldn't go for any older HDD with less than 500 Gb. Knowing what a pain in the ass was to sync it two years ago I wouldn't want to do it again when the chain would be twice the size. Cards are also silent, unlike old hdd drives, but maybe in a few yers, we can all switch to SSDs, a 500gb evo is already under the 100$ marks.