Most of the testing is for profit and to CYA from lawsuits. A good doctor doing an excellent history and excellent physical examination will elicit as much or more information about the cause of the sciatica then the MRI will. However if you need the next step, no specialist will move forward without it.
For non-insurance patients, you can get a lumbar MRI for about 600 bucks cash....be careful though because you do not want an open MRI for spine because of the clarity of the pics will miss lots of things.
Not having insurance is a choice that alot of Americans have made because of the inequities in the system. Unfortuately, when you are sued for malpractice, if the patient did not have insurance and you opted not to do SOP, it does not shield you from a multi-million dollar verdict.