There are two types of charges for email verifier feature. 

One is them is the MassMailer Email Verifier add-on license which costs $4.99 per user per month or $53.49 per user per year. 

On top of this there is an additional cost per email verification done successfully. If any of the email status is unknown that is NOT billed.

Following is the tiered pricing for email verification for the emails.

You will be billed end of the month based on the emails you have verified in that month.

0 – 250K
$0.01    /email verified
250K – 500K
$0.008 /email verified
500K – 1M
$0.006 /email verified
1M – 2M

$0.005 /email verified