This is a monthly plan with $8 (€7.50) per GB*, $8/GB for data used after that + unlimited data after $64 (€52.62) paid in a month. When the balance falls below 100MB, 1GB of data is automatically added to your line for the same price.
Ordering the GoFi line, you also get a coupon code for free 20GB/10 days. You can use this coupon anytime during 1 year.
*Bonus data is not a case for enabling Unlimited data mode.