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.
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