Only cheaper if, again, you're using 'used' components. A good basic RAID box (5-6 drive) is about $800, and that is around the same cost as a good multi-PCIe (several x4/x8 slots) motherboard, multi-core cpu, ram, power supply, and case cost. Start adding multiport SATA cards in those PCIe slots, and the cost rapidly exceeds it. BTW, that 'target' crossover point (by my calculation, with drive costs) is around $3K. Below that point, the NAS boxes are cheaper per/TB, above it, unRAID continues it's cost curve downward.
I've read a lot of the threads here on the unRAID protection scheme (having the single parity drive) and would say that if the system has an Achilles heal, that's it. A 'standard' RAID array using stripping with distributed (or multiple) parity (RAID5 and above), is a better system overall (with modern drives) than the unRAID, but that's perhaps just my opinion (but echoed by many here that would like the option for multiple/distributed parity added to the unRAID system). On the 'plus' side is the ability, matched only by the other Linux variants and the Drobo systems, to utilize a 'mix' of different drives and such. But, drive slots, even as plentiful as in the unRAID, are still a limited resource that needs the maximum utilization. So having a 'matched' drive set is just not that big of a deal when 2TB drives are <$180 ea. (!).
Again, it all makes more sense if one is hacking together a collection of used parts, drives, and the like. It's been my experience that at the end of the day, any savings one realizes from such an approach is fairly rapidly lost by time, effort, and money replacing bad or variable components. But hey, that's me.
The biggest plus of unRAID, that of being able to throw together a collection of odd drives, though, is rapidly disappearing. On might get an 'inner glow' about getting some use out of some drives one originally spent a dear amount of cash on in years past, but those ports aren't infinite (or of no/low cost) either, and it's a choice as to utilizing them or replacing them with newer/low-cost variants.
Slice of my 'cost' calculations:
W/2TB drives:
NAS 'appliance' - 16TB RAID 5 (10 drives) = $2840 - $177.50/TB
UnRAID - 16TB data drives + Parity/Drive (9 drives) - Base unit: $1400, + 9 Drives ($1620) = $3000 - $187.50/TB
That's the 'crossover' point, or thereabouts. The 'appliance' cost curve is 'flat' at that point, more storage costs exactly the same, whereas unRAID continues to fall towards around $150/TB at the maximum supported drives (but of course is more expensive at the smaller end of the scale, IF one builds out the unRAID to support more drives/ports at the start, but the differential with more drive slots/sata ports is really minimal).
Oh well, the part that worries me the most, again, is potential failure, and the grinding away of rebuilding a drive in the array. That's why the (perhaps) excessive attention to the parity drive in the unRAID, whereas with a RIAD5 array, that grinding and thrashing is distributed among all the drives in the system, and the bottleneck is distributed rather than relying on a single drive/port.