Sorry, I don’t see the distinction. “legal” only enters the picture if one side or the other disputes something. If you pay someone to do something and they refuse to deliver - that’s here the contract law comes into play.
Unless the OP is asking about criminal charges… “voilates the law”?
Surrogacy itself is not against the law.
Paying people to have sex? Usually it’s solicitation that is illegal, not paying for the act. How would you advertise? Whate exactly are you paying them to do? If they can satisfy your requirements with a turkey baster and no physical contact, how illegal is it under prostitution laws?
The purchase of genetic material - sperm and eggs - probably is either heavily controlled, need dotor license, or illegal except in special circumstances; if you choose to buy the eggs and sperm and find cheap surrogates. However, do it in a friendly third world country and the local authorities would not be able to do much.
The adoption is easy to make legal. Again, use a third-world country. the most interesting question is what happens if you try to move back to the USA, and the nosy authorities believe you bought a baby. With the right lawyers, propbably othing; since it seems to “cost” tens of thousands of dollars in “fees” to adopt a baby privately nowadays, and it’s pretty legal. If you can aford the rest, you can afford to make it legal.
the other question would be legal citizenship rights. Does a baby automatically get US citizenship if born abroad to unspecified parents and then adopted? Where does the donor parentage fit in all this? Do you have to disclose? I assume there’s a process for expediting the situation, since Madonna and Angelina Jolie seemed to manage OK. Alternatively, what paperwork do you need for a child to be born in the USA and then adopted?
The biggest problem is keeping things hush-hush, I bet. If yo attrct the attention of the authorities, I’m sure they will be happy to find someothing, somewhere, they can charge you with while they confiscate the children if there is even a hint of problems. Hmmm… you have some reclusive rich guy with a large collection of children that he bought and paid for. What are the odds the authorities could find something to mess up the situation? Family services only needs reasonable grounds to decide the children are at risk.