This is a small nuance but insurers don't bill the insured.
An insurer will issue what's called an Explanation of Benefits (EOB). The EOB will have an illustration of the billing, including:
a billed amount,
a negotiated revised billed amount (also known as the "contracted rate" or "allowed amount"),
then cost sharing with an insured's cost share based on the specific plan,
your deductible and out of pocket maximum accruals.
Generally, your deductible, unless your plan is grandfathered which is rare at this point, will capture any spending by the insured toward covered benefits. In your example the $50 would apply toward the deductible, assuming an in-network provider.
If this was an out-of-network provider, you would also potentially owe the $150 (the difference between the $200 billed amount and the $50 insurance carrier determined allowed amount). That $150 "balance bill" won't be considered by your insurer for any cost sharing accrual, generally.
Part of the contracts that in-network providers agree to includes a prohibition on balance billing; making it a non-issue.