Except it's virtually impossible to find a CPU that supports ECC.
I've used ECC in every one of my machines for the last decade or so - except for the very last once since it's was impossible to get without spending a crazy amount of money (xeon processor).
The low end E3 Xeons (-1220, -1230) are actually rather competitively priced versus the i5 / i7. However this still requires use of a workstation / server motherboard for ECC support.
It's not virtually impossible. It may be virtually impossible in crappy cheap boards from Taiwan, but if you're buying cheap parts then you're likely not making money off the results that you're producing.
At the moment, the second fastest Xeon E3 (of those with integrated graphics) is $15 cheaper than the equivalent i7. They're not price-competitive with the i5s (which lack HyperThreading), but the E3 processors have been very competitive with the i7s since their introduction. The only downside is they don't get put on sale the way the overclockable consumer parts do, and they aren't widely known.
I've used ECC in every one of my machines for the last decade or so - except for the very last once since it's was impossible to get without spending a crazy amount of money (xeon processor).