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.
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