Why not just use amazon web services(aws) and host a file sharing server there? Its free for the first year and u dunnid to pay for electricity
But you don't get transfer rates of 100MB/s.
If you don't mind size and want to save cost, can grab those M-ATX boards plus a intel pentium CPU. Should be much cheaper overall.
My current setup is a Pentium G620 + 2 WD Greens. Roughly 40W idle. Assuming you have a NAS that is 10W idle 24/7, one year you save only like $50. So you calculate bah.
Of course if size you want it to be very small then different story.
Best Value for $$$ around USD200
But you might want to addon USB3 HDD and USB 1Gbit LAN.
Otherwise if you don't mind slightly weaker CPU can try
Can addon 2.5 HDD but LAN is 100mbps only so you still need to add USB3 1Gbit LAN.