I am really sorry about your poor cat!
An MRI may give you the definitive diagnosis and location of a severe spinal cord lesion, but it doesn't sound as if you could afford the MRI, or the neurosurgery to repair this kind of damage. And, most of all, for
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Optimum magnesium gluconate results spinal cord surgery must be performed within 48 hours or so, from the accident.
With spinal cord injury, treatment includes high doses of anti-inflammatories, for a long period of time and holistic therapies if possible, such as acupuncture. You will need lots of patience, because nerves regenerate very slowly (at a rate of approximately 1/8 of an inch per month). It may take many months of treatment, and sadly, the treatment may not be effective. Only time will tell. Your cat will also need a lot of nursing care.
There are veterinary physical therapists, that you can take your cat to, but you may be able to do physical therapy by yourself at home. I googled pet physical therapy at home and came up with a number of web sites including the following one:
ezinearticles.com/?Physical-Therapy-for-Cats&id=500048
There were quite a few pet physical therapy sites and I would do a bit of research.
If you are willing to do lots of nursing care, physical therapy, and have the patience, it is worthwhile giving your cat a chance. If it turns out that your cat will never recover, and has poor quality of life, than you may have to make a decision.
Lastly, if you have not done so already you may be eligible for Care Credit. which is a low or zero percent credit card only to be used for veterinary or medical bills.