I'd be more concerned with why they're needing better pics of the heart than a one in a billion chance of autism...fyi all babies have a chance of getting autism and ultrasounds and vaccines aren't always to blame. What was up with the pictures of the heart that dr needs better ones? If you're concerned ask him why before assuming. He may have a legitimate concern.
I think there is still a lot of debate on whether ultrasounds harm the baby or not. My concern would be the insurance covering it. Most insurances only cover 3 ultrasounds per pregnancy, one in each trimester, unless there is a medical need for extra ultrasounds. Since your baby is perfectly healthy, I would call your insurance company directly and ask how many they cover. Your doctor could always be wrong and like you said doing it for the money. You don't want to end up with a huge bill later on! But if yoir insurance does cover them all then I say go for it. It is so wonderful to see your baby through ultrasound!