What's your end goal? Price? Power consumption?
You can easily put together a PC capable of running FreeNAS for ~$250-300, not including the cost of hard drives. At a current price of $150 for 3TB drives, figure out how much storage you want, what configuration you want it in, and there's your price. For example, a 4x3TB RAID-Z1 will cost you $600 and give you 9TB of space, or you could spend $300 on 2x3TB for 3TB in a mirrored configuration or 6TB in a striped (non-redundant) configuration. You'll have to sacrifice footprint/form factor (you'll be getting a mid-tower ATX system for that price), but compared to prebuilt NAS solutions running $400+ without disks right now, FreeNAS seems a better deal cost-wise and functionality-wise.