The prime reason I am buying this set is for motorization and I also love the F-150 as a vehicle. I do agree with you that the price even at a discount for the size of the set is expensive, but the set (In my opinion) is great and that is why I will buy it.
According to Stud.io, the wheel arches from 42096 do not fit with the wheel arch extenders (71689) from 42125, so there may well be a new piece involved, or they are attached differently. Can some-one with the parts in real life verify?