I bought a used Antminer S9 from Ebay last year, and performed the mods from CryptoCloaks' guide (linked in another comment here). I also flashed BraiinsOS and detuned it to 400W so I can run it in my office while I work. It heats the room enough so that it's noticeable but not uncomfortable, but I know it'll never be profitable. It's just about the experience for me. All in I probably paid around $350 in total, including mods.