I just bought a brand new HTPC and it was £277 all-in. Was suprised at how cheap you could get what is effectively a good spec PC for.
That was for ATX motherboard, case, PSU, 3.3Ghz i3 CPU, 4GB DDR3 and a 2TB hard drive. Hasn't arrived yet but I'm just planning on running W7.
Remember that only 2nd Gen Intel CPUs support HDMI 1.4 if you need that.