Hi Adrian and welcome to the MS forum. I don't remember seeing you here before. I'm glad you found us. You ask one heck of a good question - my guess would be that they want to know the total involvement of the MS in your CNS. This will give the doctors a good baseline to follow you, your care, and the success of the disease modifying treatment you should be on. I would ask your doctor that very question - is the spinal series really necessary.
You're right, even with insurance co-pays these tests are expensive. I hope you and your doctors reach an understanding.
My best, Lulu
If you can't afford the spinal, and they already think you have MS, then go ahead and start paying fo the medication rather than the diagnostic - that's my opinion. If your neuro is worth his pay, he'll recognize spinal symptoms and treat you for them, even if you don't have a spinal MRI done.