Billing companies use special codes (ICD-9) to submit bills to insurance. Sometimes, they will enter the wrong code, which results in your claim being rejected. (That happened to me when I took my daughter to the ER for a concussion. The billing company entered the wrong diagnosis and treatment codes, so her claim was rejected. Once I found the correct codes and submitted them on appeal, her claim was completely paid for.) So start there. You can look them up online.
Also, call other ERs and find out what the cost is for the same treatment, with and without insurance, then if there is a discrepancy, use that information in your appeal. Also, request your ER's assistance in getting the appeal through. The ER wants to be paid as much as you don't want to pay this bill, so they should help you, or perhaps even reduce some of your costs. Appeal the decision as many times as you are allowed. If the insurance company rejects your appeal, request an ombudsman.
If you lose, make sure to set up a payment agreement with the hospital and get it in writing. Many people think you can just pay five dollars every month for medical bills and that it's a valid payment, but that is not true. (Failure to pay a medical bill can really harm your credit, so if you need credit, it is in your best interest to work out a payment plan with the hospital.)
Good luck! Hope you get a better outcome! 
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