The actual amount you pay for usage above your monthly allotment varies per package. See the internet section of the main site here:
Ensure you've selected your community and if it's personal or business at the top of the page, select Plans and Rates, then select your package. The description will have "Additional usage " and the amount per gigabyte.
Allowances are based on calendar months, not your billing cycle. If you go over your allowance for a calendar month, it will appear on your bill in the following month under the “one-time charges” on your phone bill.