I am concerned at the expenses of the process for the parents and or caretakers. I think $200 total cost is enough. See autismenlightenment.com.
A board certified Peditrician will do the physical part.
Most of the major metropolitan hospitals offer the gold standard in Autism diagnostics and follow up programs. There are specific tests that are performed by a multidisciplinary team of professionals who can most accurately diagnose and provide a comprehensive evaluation that takes many weeks and many types of tests are performed. If you want a conclusive answer, that is the way to go. Otherwise, an individual pediatrician, psychiatrist or even nuerologist may have a different opinion. They often have funding to assist families who lack funding to pay out of pocket. Oftentimes, smaller institutions do not have funding. I think it takes a little more than an individual professional's personal opinion. You need to have the right testing done that looks at your child's entire medical and academic history as well as performing all of the best diagnostic tests, not just an assessment of current behavior.
Big name hosptials with an Autism diagnostic and treatment program often have funding to assist people who need help. These are hospitals in big cities like Chicago, IL or Springfield, New York City, etc... The big hospitals have the right testing done by a team of professionals that no individual or school staff person could debunk or devalue.
I first went to my daughter's pediatrician, to no avail. Then after a friend and parent of a son who was also diagnosed this year, suggested I have her tested by the school district she would attend. It was the best suggestion and FREE! The Team was made up of the district's Speech and Language Therapist, The Diagnostician and a Licensed Specialist in School Psychology.