No, I haven't ever bought a NAS, just built a couple. My current NAS at home is one of my sig rigs. I'm running 2 WD Red 2TBs in Raid 1. I pulled together some used parts from my closet and snagged some newegg deals. I think the total cost was about $350. That's not including the power supply etc, that I already had. If you feel adventurous, you can build your own.
You can also look on anandtech.com and see some of his NAS reviews which are helpful. Usually NAS systems + Hard Drives cost a lot. If your gonna use old hard drives, test them first then use em. If your gonna buy new ones expect to pay a lot more than $300 for a total system. Disk less NAS systems on newegg can go $300-500 and higher if you buy a system with hard drives.