How does the pricing work for internet booster packs

Hello,

How does the pricing work for internet booster packs? I have exhausted my 3G just 4 days into my billing cycle and cannot complete work tasks at the pitiful 2G speeds.
I have used the booster packs in the past but very close to the end of the billing cycle.
If I exhaust the 3G allowance on the booster pack, does the speed just revert back to the 2G in my original unlimited pack? Or, am I charged according to a tariff connected to the booster pack? Please can you confirm. Thanks.