In addition to the cost / TB of the drives, it might make sense to consider the cost of SATA ports and case slots. Once you run out of those you will have to replace drives with larger ones. I say that having done so many times over the years since I have a small case. However, no regrets since I have just re-used the smaller drives for making backups.
My re-purposed motherboard has 6 SATA now, so I'm left with 4 for storage after I use 1 for cache and 1 for parity. I think 16GB of max storage would get me well into the future, plus when I do upgrade the mobo/case I plan to make it a minimum of 8 SATA ports/slots.
I don't have a "budget" for this set, other than I want to keep the cost low and I was wanting to stagger the costs over time by slowly adding drives, reusing hardware and such. My sorta-plan is this...if unRAID is capable.
1. Start with a 1x4TB server with my old hardware, no parity, as I test the waters.
2. Add a 4TB parity drive and SSD drive down the road when I have some funds available (or would doing this be a nightmare to reconfigure?)
3. Add drives as needed to build the array.
4. Upgrade the CPU/mobo/case when necessary
Also, as drive sizes come down in the future, say I have 4TB drives with 1 4TB parity drive, can I switch the parity drive to a 6TB drive and start adding 6TB drives as I build the array, using a mixed bag of sizes like that?
Might be a little tight depending on what you do with it. If you only use it for apps and don't cache user share writes it will probably be OK to start with. I have 120GB but only use it for apps and it is only 40% full.
From that, I take it that having an SSD will improve the performance of my server/applications? 256GB 850 Pros are sub $100 now I think, so that seems reasonable if I'll get some performance in return for adding one.