There is no extra cost for your Patients to use metaMe other than the session fee you can choose to set for them.
The price you pay will depend on the plan you have selected. To view an up to date listing of the priced look here. You are also subject to a credit card processing fee for your monthly subscription payment of 2.9% plus $0.30 per transaction. This will automatically added to your monthly payment and charged to your card on file.
To identify which plan you are on, login to your account and navigate to Settings>Change Plan. You will see at the bottom your plan is highlighted in blue.