I think that surrogacy is a completely unselfish act whether a woman is paid or not. Personally, I don't think that doing it for pay is right, but I don't criticize those that do. As for using the example of Sarah, Abraham and Hagar the problem was not the surrogacy as much as it was circuMVenting God's plan. He specifically said that Abraham's child was to come from Sarah. Rachel and Leah did the same thing yet there is nothing negative about their situation. With that said, deciding to become a surrogate should be done with much godly counsel and prayer. It would not be an easy decision and any woman considering it should be prepared for all that it entails
As for "just adopt" vs fertility treatments, adopting is not cheap nor is it easy! Yes, you can adopt through foster care but you must be prepared to have the child in your home for a time and possibly have to give them back to an abusive, neglectful drug addicted parent that takes a few classes and it deemed "rehabilitated". I have a deep respect for anyone that does foster to adopt (and we may do it in the near future) but it is not easy. And international adoption costs are very high, most starting at $30,000. Unless you are in a position to adopt a child that is severely disabled then the costs will be lower. To adopt domestically is, on average, $20,000. Even after spending all of that there is not guarantee that you will bring a baby home. You can support a birth mother financially during the pregnancy, pay her medical costs and she can still change her mind. I know as it happened to us. When it is over you are left without a child or the money to try again. And if you want to get technical these mothers are getting "paid" for their child. How is this any different than paying a woman to be surrogate.
Fertility treatments can be just as expensive yet you do not have someone deciding whether or not you get to leave the hospital with your child. This is the route that we will be taking next. It is with much prayer and counsel that led us to this decision and there is no scripture stating that this is wrong. It is easy to say God opens and shuts a woman's womb, but in reality disease does that as well. I don't see God having a problem with me using a dr to help the eggs in my ovaries bypass the tubes (that are blocked by a disease) and place it into my uterus. Just as God would not have a problem with me using a ventilator to keep me alive even though he is in charge of life and death.