It definitely costs more to roll your own, than it does to buy already built. However, if you know what you want in a frame, and you know what you want in a group, building your own may be the only way to go.
I built my road bike up from a bare frame and components three years ago. Found the frame and wheelset on ebay, group and bars came from an online bike shop - new groups weren't any cheaper on ebay. Since then, I've replaced the handlebars (found Cinelli RAM bars cheap on ebay), wheelset (who doesn't buy another wheelset every so often) and saddle (several times, I got a picky butt), but otherwise no changes. No need to, I got exactly what I wanted, and the bike has been trouble free for three years. The only components I changed were due to good deals I found later or just because I wanted them, not out of necessity or deficiency.
Besides, there is something neat about building your own bike - the end result is uniquely you.
Nothing wrong with 105 - a riding buddy has been using 105 for the past three years, with no problems.