Have you ever visited any flea market nearby your house?
Last time I visited one, I saw tonnes of used (but still working) smartphones. I wondered if I can use these cheap smartphones for mining?
The answer is yes.
I found a lot of great smartphones for 5$ each and the seller told me that he'll sell me for 3.5$ each if I buy more than 100 old smartphones.
That might be a good investment but who knows...