My QNAP was around $400.. then you have to buy the drives (I bought them a few years ago.. they are cheaper now). Synology & QNAP are the bigger brands in the market. I went with QNAP because i liked the interface.

With both QNAP and Synology you can decide how much 'CPU' you need. If you are just using storage you can get the ARM CPU's. If you want to do Plex or VM's you need the faster Intel/AMD CPUs.

Just search Amazon (B&H Photo also has good deals every few months). Around the holidays you'll find the best discounts.

If you build your own all you need is an old PC that can handle 4 hard drives. Then you just need to buy the HD's and learn how to set it up (the internet has everything you need 🙂

If you are in the SL/UT county area I have a 4U server case you can have for free (i probably have a mobo+cpu also laying around).