Depends what you need - you can pick up older gen Xeon chips for cheap and the performance often isn't that much worse than modern consumer grade stuff. If you're looking to build a consumer-level NAS or home server, Avoton is pretty cheap and takes ECC RAM.
Unfortunately, Avoton might just suddenly stop working on you.
https://www.servethehome.com/intel-atom-c2000-series-bug-qui...
Some i3s support ECC:
7th gen: http://ark.intel.com/products/97125/Intel-Core-i3-7101TE-Pro...
6th gen: http://ark.intel.com/products/90731/Intel-Core-i3-6300-Proce...
It should be 1/8th more, plus a bit for the scrubber. But in practice ECC memory is "enterprise priced" so it's more like double.
How they get you is Intel will sell you a xeon which is the exact same die as an i5 in a different package for more money.