Can someone please explain why my insurance company won’t disclose this info?

I asked my health insurance company to inform me of the “allowable amount” of a service. They refuse to provide it.

The provider charges a percentage upfront (my coinsurance). I am so sick of the coinsurance being based off the provider’s grossly inflated bill, just for my insurance company to come back and say the allowable amount is a fraction of that. (Eg.: I saw a specialist who billed $1700! Allowable amount was $400. 20% of $400 is way less than 20% of $1700. It’s such a hassle being forced to pay the provider’s amount, then have to beg their billing office for a refund that takes 60-90 days to get.)

The provider office secretary said to call my insurance company and verify what the allowable amount is. If I can find out the amount ahead of time they will only charge me coinsurance of the allowed amount. But, now the insurance company will not provide it.

Is this legal? I pay for insurance. They know how much they’ll pay because it’s a contracted rate. Why won’t they provide me the information???