If you're going for bottom of the barrel, off the shelf tends to be cheaper, especially if you go with a legit Windows license (assuming you aren't just using Linux for the media PC).
Building gives you better ability to pick and choose parts. The guts of my HTPC cost under $300 when I built it a few years ago, but I already had a HTPC case with drives in it (and have since put an SSD in, tried a few different video cards, etc).
'course, there's also the middle ground of snagging an old machine like a Core2Duo or whatever, which are still well above what you need for video (doubly so if you have hardware acceleration for the formats you're watching via the video card).
If I recall your earlier posts about this, your HTIB receiver has optical-in? You could probably use something like
this on the laptop for audio, if your TV doesn't have an audio input for the VGA cable (in which case you could just use a cable like
this).