The only option that I know right now is the Asus Q87T board + Intel Core i3-4130T 35W processor. But this combo is not only expensive but overkill for this application and it generates a lot of heat too.
Does anyone know of such a motherboard available for sub-$200 ? I would also consider AMD boards.
Do you mean sub-200$ for the motherboard + CPU combo? Because I found the Q87T on several sites
for just 150$.
I couldn't find any motherboard which is cheaper and fits your requirements (all manufacturers).
You can save by buying a cheaper CPU like the Intel Pentium G3220, even if it has 54W TPD (higher than yours), it will use less than an i3-4130T with 35W TPD while idling and under light load.
See here for some advice on the Haswell CPUs: Optimal Haswell CPU for low idle power HTPC/Server
I don't really know about low-power AMDs, I can't come up with an advice on them, maybe somebody else here?
I'm also considering building a NAS myself but I can't recommend you something as I don't need mSATA.