I think it is best to omit this expression because it has no equivalent in English. "Vientre de alquiler" is an absolutely standard expression in Spanish for surrogacy. It is almost inevitably mentioned when the subject comes up. Most Spaniards, I suggest, wouldn't have a clue what you were talking about if you called it "subrogación". English speakers, on the other hand, know the phenomenon as surrogacy, or surrogate mothers, and don't call it anything else, in my experience. So no expression of the "womb for hire" kind can be regarded as a translation equivalent of "vientre de alquiler", because it's impossible to use any such expression in English without it sounding jocular and/or pejorative, which is not the case with the Spanish expression.

So if you put surrogacy, nothing else is needed in English, whereas if you put "subrogación", something else (namely the everyday term "vientre de alquiler") is needed in Spanish. And anything else you put in English will inject an inappropriate note that is not present in the Spanish. Unless you put a formal synonym like "commissioned pregnancy", and what's the point of doing that?

Of course, if you follow my advice you may want to add a note to the client explaining why.

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Note added at 2 hrs (2017-05-16 13:47:02 GMT)
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Sorry to labour the point, but the more I think about it, the more convinced I am that this is insoluble.

The author has added "vientre de alquiler" because although he may be writing for specialists who would understand "subrogación", he/she and they are aware that most people wouldn't. So he's effectively saying:
"subrogación, or vientre de alquiler, as people usually call it"

But in English it's commonly known as surrogacy, so in

"surrogacy, or X, as people usually call it"

there is no possible value for X: there is no other expression for this that is common, let alone more common than surrogacy. Anything you put, and certainly any of the suggestions here, will be contrary to the author's intentions.