I've been building inexpensive PCs with Gigabyte H81 ITX motherboards, LGA1150 Pentium G CPUs, 4GB RAM, 120GB mSATA drives and Rosewill ITX chassis. I can build a whole machine for around $250. The chassis will still have room for an optical drive and a pair of hard disks, should you want them.
I specifically like the Gigabyte board for having both mSATA and mini-PCIe slots, plus the cutout to add antennas for 802.11/bluetooth. There's just a lot of flexibility for an ITX machine.