A hard drive may be cheaper than a high-end graphics card, but I don't think they are comparable in terms of ROI.

If Chia's value is a never ending upward rocket then yes, you will see people willing to spend more and more money on plotting hardware and electricity to run them. If at some point Chia's value stabilizes then there will be some point beyond which it makes no sense to make those investments. There is also the hope that Chia catches on and the general public farms some plots on the computers and the extra storage space that they already have. If this happens, it would quickly push the ROI on buying plotting hardware down because the netspace will be immense and each additional unit of storage you buy will be too small compared to the total netspace to appreciably improve your chance of farming new coins.