Most baby-buyers seem to prefer a separate egg donor and surrogate mother.
A mother carrying her own biological child is considered too much of a risk for refusing to relinquish her child at birth.
Plus, luxury baby-buyers tend to want eggs from young, pretty, intelligent women (hence college students being targeted by predatory ads) but they want the surrogate mother to be old enough to have a proven track record of carrying at least one child to term and to live a low-risk life where the paid pregnancy will be the priority, not a career or a university course (or a social life).
In the US, Military wives with deployed husbands are targeted, as they are often stay at home mums for their own children living off one modest wage.
Plus they aren’t having sex because their husbands are away and lots of baby-buyers want surrogate mothers to sign contracts stating no sex.
Alternatively, a white, college student egg donor and a poor black or brown woman in a developing country is a way to buy a luxury baby on a smaller budget (plus laxer laws mean more embryos can be implanted eg the Irish men who commissioned triplet girls in Kenya)