looking at atx and ECC on a local price checking site, I think you will not get it as the starting costs is easily twice your budget.
Will have to be looking second hand I suspect.
You are mostly right. I got a new ASUS M5A97 board (for USB 3.0 and a bunch of 6Gbps SATA ports) with a coupon and credit for about $83. For those willing to gamble, Newegg has a few AMD boards with ECC capability (mostly ASUS) refurbished or used for less. I didn't try to save money buying used for obvious reasons, but it's doable.
For those who don't want to make their own file server, Newegg has the Fujitsu MX 130 S2 micro server on sale today for $140 after coupon Fuji91112US Should be good for 4 HDDs, 2GB DDR3-1333MHz ECC memory, and an energy-efficient PSU, but lacking in USB 3.
Btw for those who laughed at my cosmic ray comment, read this:
http://lambda-diode.com/opinion/ecc-memory I won't need to replace my gaming rig for a while, but as processes shrink more and more and become ever more susceptible to soft errors due to lower voltages, I am betting that eventually all memory will be ECC, not just server memory.
good luck, ive been planning my nas on a esxi server forever, went from (I'll just use a cheap z68 mobo!) to (holy shit, vt-d gets expensive! lets just make the nas) to (fuck maybe I should get a supermicro board) to, (nah too much money for the nas) to (fuck it i quit)
still don't know what im going to do. I have a asus z68 board gathering dust. if im going to step up to supermicro($160-200) then I need more expensive memory($80 for 8gbs?) + either a i3 2100 base($110), or a freaking i5 2400 to support VT-D($180, which is more than I paid for my freaking 2500k) or a Xeon E3-1220 to support the ECC properly ($209)
If I were you, I'd just sell your old mobo for whatever you can get, and buy the N40L or something like that. It's hard to build a ECC-capable file server for cheap, as I discovered the hard way. And that's just for NAS/HTPC, not for any virtualization or anything.