A NAS is great for your backups of e.g. your valuable log data and so on, but you can't call them cheap nor can you do what you like with them.
If you sill have a usb hard-drive lying around or a Beaglebone Black that you bought to play with but without a destination you're halfway there already!
Because here's a fantastic home-brew project to bridge the gap: NAS = beaglebone black + USB HDD
check it out: http://joshnock.com/post/BeagleBone-NAS-Server