From my understanding, the P8P's video boost can't be done in the phone itself, but it's actually uploading the video to Google's servers, processing it there, then downloading the finished product back again. In fact, it probably uploads two videos, one at high ISO and one at low ISO to get the full HDR range.
For anyone who wants to take lots of videos, this sounds like it'll blow through your data caps very quickly.
In Canada, we have "unlimited" in that we have service like 40 to 50gb, and any data after that is reduced to speeds "up to" 512kb/s. Plot spoiler to no one, those speeds aren't definitely not 512kb and it's pretty much unusable at that point.
So no, we don't have mainstream carriers that offer actual unlimited for under $100. That said, I have 40gb on my plan for $50 CAN/US, and it's sufficient. Don't think I've even come close to consuming that.