Not necessarily an opportunity cost. Often they are paid in a more valuable commodity, i.e. visas.
But I'm writing from the perspective a grant reviewer. Systematically abusing immigrant labor.... well, it isn't exactly a more charitable interpretation. So, from my perspective, a bit of a moot point :)
(There's also OPT, btw, which is critical in the process, and H1B lottery chance, assuming a US Masters degree, was more than 50% last time I looked at it a couple years ago. O1 (and even EB1A/B) are not that rare for well-published PhDs, and EB2-NIW is quite easy for any PhD worth their salt, I'd say.)
You can master-out in the middle of a PhD program, which is frowned upon, but it is difficult to get funding if you directly apply for a masters degree.
Perhaps the solution is to decrease the negative cultural backlash against predisposed intention of leaving with a masters despite applying for a PhD, which is currently seen as some sort of betrayal of the adviser if you are going to have your grad students do boring work. It'll be a win-win deal in that case.
When I was in school, one applied to "MS/PhD" programs most typically. And all imaginable variants existed in terms of funding and likelihood of leaving with only a MS. Some schools would fund no one till they finish the MS stage, and others would fund everyone from the beginning (just give every accepted student a full fellowship for the first couple years). And also some schools were pretty notorious for kicking people out at the end of the MS stage, with a difficult exam at this step or not enough advisors to take them on for the PhD.
One positive side, though, is if you wash out of a PhD program at any later stage, you at least walked away with a Master's.