It would seem the price eventually gets pushed up much higher in total when you consider you'd have to add extra components before it will serve it's purpose. I don't think I'll go this route unless there is something I'm missing.
If cost is an issue, the HTPC does not need to be cutting edge. Built mine with old parts that was laying about - A H61 board, running a Celeron 2020. Threw in a GT520 graphics card (as I had upgraded my PC and the 520 had HDMI out) and 4 gig ram. Added a wireless dongle and it runs XBMC (12.2) on Win 7 (The Linux version starts quicker but I wanted the Windows functions for browsing and a game or 2 for my son) I have a separate "server" with all my content in shared folders, and can play anything, including 12 gig full HD movies (without buffering). Before this I had an even older G41 board which gave as good results.
You are probably better off spending your money on the "server" side; even the most basic pc will be strong enough for the HDPC, as long as the graphics is upped. I regularly have the HDPC in the living room on my wife's series, while I watch a movie on my PC in the bedroom (fixed cat5e connection from server) The network runs through a standard Telkom ADSL router (Netgear N300 - but I run my wireless at 145) Had a 802.11g before, but this did cause buffering every now and again.
Moral of the story - its best to separate HDPC and server - my server currently runs 7 Hdd, and I'm planning more. A single disk on a small HDPC tower will probably run out of space within a small timespan, and is not really conducive to upgrading cheaply. Its position is also not normally good for proper ventilation and wiring. If like me, you later add viewing areas (lounge, bedroom and laptops) the server route makes more sense.
Forgot to mention, bought a cheap R50 buck remote over e-bay as I could not get one locally, and my tower did not come with one.