This is a question that we (as parents who have adopted our children) sometimes get asked. It really offends some adoptive parents and makes them very upset. I however see things differently and here's why...
Let's look at this question from a different angle. When we pay money to our adoption agency (and all other related agencies) we are not buying our children - we are not putting a dollar amount on them so to speak. We are paying for the services that our adoption agency provides related to adoption such as orphanages, social workers, foster mothers, child care, health care, agency overhead, birth mother counseling, etc. which ultimately bring our children to the point where they come home with us. These adoption agencies are not getting rich from the money we pay them. This is similar to the fact that you don't buy a biological child from the hospital where he/she is born. The child's parents (or insurance) pay their bills for the services that the hospital provided during the birth, and hospital stay for both mother & child. This is what I will explain to my children when the need arises (when they are old enough to understand). I think they will be pleased with the way this adoption fee money is used, and will understand that you can't put a dollar value on any human life - whether it be a biological, or an adopted child. So when someone asks "how much did adoption cost", they don't mean what dollar value do you think your child is worth. My children will be able to make this distinction, and your children could as well if you explained it to them.
Speaking of stranger's adoption questions in general - there are still in this world more children who need forever families than there are families who are willing to adopt these children. This is despite the fact that millions of fetuses are aborted (killed) each year. Since this is true why not (when given the opportunity) say something positive (or informative) about the adoption process when someone asks you a question about this subject? I mean this person could take this information you gave them and look deeper into adopting a child themselves. Maybe they will even eventually adopt a waiting child that nobody else wanted. Maybe they will pass this information you gave them along to someone else who will use it as the foundation for a similar journey. This is how things like this get started - as an idea. You could provide fuel for this idea and make it grow.
I know that it is in everybody's best interest that every child that needs a family will eventually find a family that wants them, raises them, and loves them