Cheapest way, plug an external hard drive into your router and share it. A lot of all in one type routers have this option. It's a bit slow but it works.
Next cheapest, use an old PC like you suggest. How well it works depends on the hardware and how much effort you put into setting it up.
It could be as simple as using windows file sharing and sharing a folder with the workgroup (not always actually that easy)
Or using a dedicated NAS OS like freeNAS (but it has fairly high RAM requirements) or Open-e DSS (a biit tricker to set up and has limitations on the free version). Or you could step it up a bit an learn how to configure a server version of Linux, Ubuntu server or FreeBSD for example.
And the easiest and most expensive - buy a NAS