I keep remembering when there was no health insurance and doctors' fees were low and we could all afford to see a doctor or go to a clinic. In this wonderful modern world we pay through the nose for what used to be affordable because doctors did not have a guaranteed income and had to woo their patients.
What you need to do is to request a copy of your results for each test when you have them done, read the results and pay special attention to the final "impression" as well as any recommendation that is noted on the report. If you have questions about what you have read then see the Dr. who ordered the test (face to face) for a detailed explanation. You do NOT need to have any test that you don't want to have. For what you had done I don't believe that you were "over billed" and whether all tests were necessary would depend on what was "seen" on the first films. Additional testing such as MRI may be necessary when the breast tissue is very dense as Mammogram does not "see" thru dense tissue and might very well miss something. In the future just demand detailed explanations of the test results so that you will feel comfortable accepting any further investigation. By the way, the price you quoted for the MRI wasn't out of line by today's standards. Regards ....