This is an interesting question. I would personally not pay extra for a modem in my MacBook, BUT IIRC, part of why Apple did not like the Qualcomm royalty structure was that it applied as a % of the total device price, not just the modem component. If so, that would have the potential to add quite a bit to the cost of a computer. If Apple can source their own modems, then it would just be a flat fee of whatever they cost.
I could actually see them doing this. Depending on the cost, it might even make sense for them to build this into all Macbooks to increase demand & drive down the cost to build. I could maybe see them making it standard on all iPads too, but I'm not sure if they will, as those already have that extra $129 (or whatever it is currently) built into the price for the cellular upgrade, and they'd lose out on that if they made it standard.
Interesting question.