I think this is a bit of a tricky question. A large chunk of this price is going to come from the tiny additions/comforts each individual wants from an HTPC. For basic playback of a 1080p video (not specifying formats or codecs for video or audio - or even taking the source into account) it really doesn't take too much to get smooth playback. My $99 Logitech Revue can play back 1080p content with very specific caveats.
1. It must be on a wired connection - even with Wireless N and only about 30 feet from the AP I cannot play it back over wireless except in a very specific and non-enjoyable case.
2. The source cannot have DTS encoded audio. If it is then I will get no sound playback at all, but for some reason I can stream it at 1080 over wireless.
There are other small devices that can playback 1080p vids with ease and use minimal raw horsepower to do so. One I'm looking at now is this one -
http://www.anandtech.com/show/4931/...ve-streaming-media-player-with-spotify-in-tow with an MSRP of only $99 as well.
The thing about the two devices I've just mentioned is that you only get playback - and not from a local source. You either need to direct these devices to a network source of some sort or bring external drives to the device. Also, some people use these things to do transcoding. I do not, but I've noticed many people want this feature. Also, a few people I've seen want their HTPC to have torrenting capabilities. Again, I do not but some do.
If you're specifically looking just to build a machine with plenty of flexibility then I'd first plot out exactly what features you need. Have a case in mind already or need one to match your current components? Does it have a PSU? Is it a standard PSU or something a bit smaller? How often will you boot/reboot this HTPC and how quickly do you want it to do so? Does that mean it needs an SSD or will a traditional 2.5 or 3.5" drive work? Do you need local storage on this HTPC and is there space in the case you've chosen? Do you need a specific OS to be running on your HTPC? Many choose a flavor of linux so they can strip it down for very quick booting and then just set it to launch straight into their media player of choice (often XBMC). What kind of audio capabilities do you need (this is where it can get a little complicated)? Will slow menus when browsing large libraries really annoy you?
if you're looking for something quick. small, and cheap as a starting point I have been poking around with the idea of getting:
mobo:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131635
case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811133093
and sticking an extra 2.5" laptop drive I have sitting around in there and slapping some distro of linux with XBMC on there. I haven't actually built the thing yet and am currently wasting a 17" MBP as my HTPC, but since I have a newer MBP I haven't been rushed to do something about it.