I have a Xeon D1521 board that I use which does support ECC. It was under $500 when I bought it, but can't find it now. Supermicro does make a mITX board around it with 2x10GbE for $800. Asrock (which made the board I have) sells a uATX one in mid $500 range (I think I spent $499 on mine[1]).

1: https://www.asrockrack.com/general/productdetail.asp?Model=D...