The biggest flaw in your argument is to "build you a PC that would run the game at max settings at 1080p". Why?360 is only $200 now. I meant build me a PC from that same point in time when the xbox 360 was still $500. It wouldn't have cost you under $800. At this point, all the tech is old and cheap. I bought a core i7 when they first came out, that alone was over $400.
The Xbox itself can't run UT3 at 1080p at max settings. It's pretty likely it can't even run it at 720p or 480p at max settings, which is a good reason why the game doesn't have "max settings" available at all. It has "the settings that run good at somewhere around 720p at somewhere around 60fps".
I couldn't say anything about 5 years ago, but 4 years ago I bought a system for $400 that could play UT3 at 1280x1024 at 60fps with max settings. Back then it would cost you $400 to buy an xbox and the game and no xbox live subscription (yes, I'm intentionally ignoring the neutered, hard-drive-less "arcade" unit).
I think others' point was that over time the investment isn't as worth it if cost is your only determining factor. $810 for a PC 5 years ago would have gotten you the kind of equipment that would probably last you until the end of this year or next year (whenever new consoles start pouring out) and even then you'd only really need to upgrade the video card and ram right away.