If I build a desktop box using parts available today for $400 without recycling anything (M/KB, OS, case, PSU, HDD, etc.) I would be left with an extremely modest set up that would not give me the same experience as either console.
At $500, I would be left with a decent budget gaming PC, but in all fairness to the XB1, that price wouldn't include the rather impressive yet somewhat pricey Kinect 2.0. $100 for a sensor/camera array is actually quite reasonable, so pricey probably isn't the best word, but when a PC builder considers the difference in parts options available for the same price, that extra $100 buys a significantly better VGA card.
For a budget PC box, I don't see why one would want to use a Kinect 2.0 camera, even if it didn't cost $100 (which it won't if MS sells them as a separate SKU; they will cost more) and even if one could even buy a Kinect 2.0 to use with their Win 8 PC at any price.
A PC builder would also take the money that would go into a BD drive, swap it with a $20 DVD drive and use the difference towards a better CPU or VGA card. You won't be watching BD movies on said PC, but... not many PC users do compared to PS3 users.
I don't really like these comparisons because I already build my own PCs with gaming as a secondary function. I prefer to play on consoles or *gasp* for a quick game, on tablet or phone. Secondly, I have absolutely zero interest in building a basement budget $400 gaming PC as my idea of a decent gaming rig would start closer to $1000 at which point you're comparing apples and oranges with regards to $400 consoles.







