As with most projects that are computer-related, cost will be directly related to a couple of things: your own expectations and whether or not your HTPC needs to be "pretty" and integrate seamlessly with your existing home theatre equipment. If you can deal with lower-end performance, a limitation or two and an enclosure that isn't overly sexy, this probably won't be that expensive of a project. For a lot of us, it could be accomplished simply by using recycled, second-hand hardware. If you're a speed freak to the utmost degree and obsessive compulsive enough to be unable to mix a beige enclosure with your black television... you'll probably end up spending more jack than you might expect.