A PC-based system will likely come in much cheaper than a dedicated NAS box, and there are open source options...
around to create a PC NAS (FreeNAS springs to mind). If you are OK with installing software and have a spare PC lying around, then it is pretty easy to set up.
Remember, even though many PCs come with built in RAID, it will generally only be RAID 1 or 0, and if a greater level of redundancy is required, then a separate RAID card will be required - which will push the cost up a little.
In summary, you can set up a PC to do the job of a NAS device but you'll get what you pay for in terms of features and resiliency and if you have a lot of data you should look at a product dedicated to the job.
Related Q&A from Clive Longbottom
Better data center and system management tools are radically increasing the number of servers to each data center employee.continue reading
The high cost and increasing compute densities in server racks make it difficult to implement a cryogenic cooler in the data center.continue reading
Funding a data center project or an expansion requires building a business case that focuses on three areas.continue reading