I don't think an exploitative relationship is implied or required here, but it's certainly a big danger (probably the biggest) one would have to be super alert

I don't think an exploitative relationship is implied or required here, but it's certainly a big danger (probably the biggest) one would have to be super alert for if this was to be performed at any scale (which exists already, by the way). E.g., I heard that Minnesota has some sort of routine in place for voluntary surrogacy, which costs the adoptive parents on the order of $100k, of which only ~25k goes to the birthing mother. (A large part of the cost is administration and health checkups/treatment/insurance). It doesn't sound right to me that only 25% of the proceeds should go to the star of the show, so to speak. But this doesn't imply that it is impossible to do this ethically. Regulation that's well thought-out, as well as a sound, pre-existing social welfare net, is critical.