The iMac 27" is my favorite.. none of the all-in-ones it's compared to are even the same size on that chart. Even if you build a computer with the same equipment it's more:
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$1100+ for the 27" IPS monitor from
Dell
$150-250 for the processor (i5 2390(I think)) according to
Intel
$120+ for the graphics card (desktop version, not sure about the mobile) according to
Newegg
Anywhere from $100 to $500+ for the tower and necessary equipment ( just depends on what tower, cooling etc. you purchase)
$180 for the Windows license
according to Newegg
Newegg
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$1550 at the minimum if you do it yourself, probably more if you buy it from a manufacturer. Slightly more expensive than the $1500 iMac, and not everything comes in a nice looking package. Plus the hours you get to spend putting it together.
I did the math for at least that one Microsoft, stick to making Windows 8 as awesome as Windows 7 instead of making terrible ads like this.