I just went through this last year. I wasn't expecting all of the foundation companies to agree on the cause and amount of foundation deflection, but I also wasn't expecting such a wide range of opinions. I got 4 quotes ranging from $0 to $11,000. Only one actually used any sort of measuring equipment. All of the foundation companies offered a 3rd party engineer inspection. My thought was, if you hire the engineer, then they don't work for me.
I then hired a structural engineer for $400. It turned out to be $400 well spent. He came out and spent considerable time measuring the foundation and looking around in the attic. He then recommended a foundation company. They put 10 piers across the front of my house, but I was not happy with the results. I just didn't see any change. I knew that things would not go back to day 1 perfect, but I also didn't see improvement. The engineer that I hired came out and measured again, then went to fight the foundation company on my behalf. They came out and did a second lift, and I was happy. I don't think the foundation company would have done the second lift had I not had my own engineer.