Whether or not a surrogate mother is paid should be a decision made by the parties involved. If a woman decides to create an income for herself by bringing children into the world for couples unable to produce their own, than she is providing great service. There are many reasons that couples are unable to have children in a traditional manner.
If the surrogate mother is providing a service, then I see no reason not to. Pregnancy takes a lot of responsibility and physical risk, and it isn't too much to ask for compensation in return, but it should be up to the surrogate and the parents involved to decide their own contract.
My wife and I have 4 children, the toll that pregnancy takes on your body, and emotions is bad enough. If you were to go through all that and not have the reward of your child at the end of it I can't imagine anyone wanting to do that. But there are thousands of couples that cannot have children, and surrogacy is the only way they can. If there is someone willing to go through such sacrifice for someone else, why shouldn't they be compensated?
Surrogacy should be seen as a donation, much like donating your kidney or your blood to save someone else. It seems very strange to have someone carry a pregnancy to term for you and you pay them for their body. Either that's prostitution or you are renting the space in her womb. Either way, it become way to easy to take advantage of poorer women as a paid job, so I think it should not be legal.