Actually this is incorrect, Broadwell-E uses the same LGA2011v3 with X99 platform socket as Haswell-E.
It is Ivy Bridge-E (4820K, 4930K and 4960X) that uses LGA2011 socket with X79 platform.
Taking a quick comparison between same quality/model CPUs and Mobos from the Netherlands (cheapest in Europe, Germany being very close) shows the following:
CPUs:
Intel Core i7-5820K - € 399,-
Intel Core i7-6700K - € 347,45
Mobos:
ASUS X99 Sabertooth - € 349,-
ASUS Z170 Sabertooth Mk. 1 - € 247,-
Together that's a price delta of: € 153,55 or $ 175,13
Now you could say I didn't take the comparison with the same mobo's of the OP... fair enough but it was to illustrate a point.
If I compare it between 2 of the same models for different chipsets I would compare these as the closest to the M5 edition he selected:
MSI Z170a Gaming M7 - € 209,-
MSI X99A GAMING 7 - € 266,50
Which shrinks the total delta down to: € 109,05 or $ 124,36
Still a big delta some people don't have and this difference becomes larger in countries other than NL/DE.
Edit:
Intel Core i7-6700K - Mindfactory.de - € 339,15
Intel Core i7-5820K - Mindfactory.de - € 381,39
MSI Z170A Gaming M7 - Mindfactory.de - € 198,02
MSI X99A Gaming 7 - Mindfactory.de - € 259,11
Even from Mindfactory.de that's still a total delta of: € 103,33 or $ 117,84
The delta, whilst worth it in general, is still there and not all can afford another 103 euros.