The fact that the Xbox One comes with Kinect 2.0 does not make up for the price.  Even at $400 the X1 is noticably weaker than the PS4 and more complex to program games for.  The truth is that Sony intentionally caught MS off gaurd.  I mean think about it:

1) Everything Sony said prior to the 2 Week teaser for the playstation event was  about how Sony is not ready for the PS4 yet and that they would wait.  In addition to this, PS3 has many major exclusives coming out this year giving more evidence that they intend on sticking to PS3 another year.

2) Mark Cerny recently talked about how Randy Pitchford convinced him to use 8GB of GDDR5.  He said what convinced him to do this was the fact that they would beat the next Xbox everything hardware wise.  I bet they realized this and saw that MS was not ready to launch or at least make any major changes to their design .  

3a) A lot of rumors and leaked documents have hinted that MS  was intending on using DDR4.  This was supposed to be launched late late 2012 but was delayed due to less growth in the desktop market and more growth in the mobile market which has no need for DDR4.  DDR4 is not as fast as GDDR5 but it can be close (and better with EDRAM/ESRAM), and it would dropped in price dramatically over the years as it entered the PC market (In contrast DDR3 prices should continue to rise like DDR2 did when it was replaced).   

3b) Sony once again probably found out MS's weakness, saw their own strengths and pounced.  MS predictably realized that if they didn't launch this year, Sony would gain far too much market share and so they made a conpromise.  The motherboard designs could be easily adapted to switch to DDR3 but GDDR5 was out of the question.  

Sony is already in production manufacturing PS4's, and MS didn't even have more than a couple working units at E3.  Sony clearly ambushed MS.

... and where does the "cheaper" factor in?

Yeah kinect is the main one.

The new Kinect is in fact very expensive to manufacture. It more than makes for the price difference. Maybe not make up, but certainly makes.

There's nothing new in your OP we all know this already.

Theory? The fact Sony got rid of the PsEye to reduce the cost by $100 is why it's cheap.

PS4 selling at a loss would be my thinking.

PS4 Eye only costs $60, PS4 was going to be cheaper than Xbox One from the get go, Sony knew that is what they wanted. They've learned a lot since 2006.

They will likely launch another sku with Knack and the PS4 Eye for $499 as well.

The eye is only $60 retail, which means it likely costs even less to manufacture.

thanks for wasting my time with a theory that has nothing to do with your title of why it is cheaper.