Looking at the PS3's first pricecut which happened in late August 2009
PS3 NPD sales went from 210k in August to 490k in September, 133% bump in sales.
Looking at Halo 3 launch in September 2007
360 NPD sales went from 277k in August to 528k in September, 91% bump in sales.
Looking at Halo: Reach launch in September 2010
360 NPD sales went from 357k in August to 488k in September, 37% bump in sales.
The PS4 pricecut isn't as significant as the ps3's was. But this isn't the first Halo on the XB1. I'm going to guess both consoles get a 30% bump from September.
PS4 - 442k
XB1 - 365k
WiiU - 75k
3DS - 110k