They Shall Not Pass Price
They Shall Not Pass commands $15 as a standalone piece of DLC content. This is standard practice for EA and it's the formula the company's used for previous DICE titles including Star Wars Battlefront.
What this means is that the Battlefield 1 Premium Pass to offers you a bit of a deal on the DLC expansions. The Premium Pass is $50 without a deal which means you'll save $10 on all upcoming expansion packs for Battlefield 1.
If you don't want to pay the full price for the DLC expansion you'll likely have to wait. We could see a few deals emerge over the course of 2017 though we wouldn't expect any blowout deals on a $15 expansion pack.
You're more likely to see retailers (and EA) offer deals on the Battlefield 1 Premium Pass. We've already seen the price drop by $10 at some places so you'll want to keep an eye out for Premium Pass deals.