You have ZERO recourse here, other than demanding an exact copy of the terms of your policy. There is often a specific co-pay involved in procedures such as CT scans. If it's listed as $500, you're fucked, and that's what you owe.
So what about the rep who erroneously told you $100? They usually have a disclaimer on these insurance phone numbers that information is not guaranteed or any promise of coverage. That's to cover their asses if some slug gives you the wrong info.
So here is what I would do:
1) Demand a copy of your policy (online, e-mail, faxed, or mailed) so you can see exactly what your co-pay is.
2a) If it's $100, no sweat. Call them and they are legally required to fix it.
2b) If it's $500 (more likely), you are likely fucked. However, call up and complain, and ask them to pull the call. Tell them that you wouldn't have had the procedure done if you knew it was $500, and you want them to cover the other $400 due to the erroneous information provided to you. If they can pull the call and find that you were given this erroneous information, they might send you the $400, but it might be a bitch of a fight to make it happen. You also have no legal standing to force them.
You should also find out what they mean by "estimated co-pay". Estimated based upon what?
If it's "estimated based upon total cost", then you're fucked. They might have estimated it would be a $800 procedure when it was really $4200. Anyway, you really need to see your policy.
I have my own CT scan horror story, from about 6 years ago.
I went to a hospital facility that claimed to take my insurance, but when the bill came, it showed that my insurance denied the claim because the facility was NOT a preferred provider! I was told to pay $3800.
The hospital billing department refused to work with me. I got a really obnoxious rep who, after hearing my entire story about being given the wrong information regarding my insurance being taken, replied, "Well, I have good news for you. Instead of requiring the $3800 payment immediately, we can break it up into up to five easier payments!" She intentionally ignored me point -- that their own employees gave me erroneous insurance information. Finally, after it was clear she was intentionally ignoring my point, I called her a "moron", and she called me disrespectful. I told her that intentionally ignoring my point and repeatedly telling me to simply make payments is also disrespectful. I eventually hung up on her.
Hilariously, after a lot of investigation, it turned out that I was given the right information after all. The hospital had temporarily dropped the insurance company for 6 months, but had reinstated their contract about 4 months before my scan. Yet their computers were still showing no contract, and it took a lot of investigation on the insurance company's part to come up with this. So I seemed like I was in great shape.
BUT...
It turned out that the "allowable charges" for this CT scan were a laughable $900, and I was supposed to be responsible for the difference. (It was some weird situation where there was no maximum, and the insurance just paid up to $900 and I would be responsible for the rest, despite it being in-network!)
I asked, "So if there's no maximum, what if they billed for a million dollars? I would be responsible?" They told me yes.
I told them that this made zero sense. After months of more investigation, they came up with the conclusion that the hospital mis-billed the CT scan, and that it should have been $1800, not $3800. This left my balnace as $900.
However, with all the confusion, the insurance company accidentally paid the hospital twice, leaving me with a zero balance. Obviously I kept my mouth shut at this point, and that was that.
See, Willie? It could have been worse.