I cant answer your question fully as I think there can be quite a few determining factors. What ended up being my deciding factor on ECC was I could use more RAM in the mother board with ECC then I could without it.
According to my motherboard specs (supermicro H8QGi-F)
I can support up to 512GB of Ram, so I'm guessing I can support about 100GB of ECC Ram. Though that is not in questions since I'll be adding less than 64GB of ram for the system.
However I'm more concern on if ECC is actually needed nowadays. I can save hundreds by using regular ram, so it's really a question about cost.
It's worth keeping in mind that it's not just bad ram that can cause errors, but even things as crazy as cosmic rays. It just takes a bit of strong radiation to flip a bit from a 0 to 1 on a ram chip, and without ECC, none of your applications would know the difference. You'd simply corrupt your data, potentially crashing your machine or otherwise causing problems. ECC can cut down on this sort of thing dramatically.