It's a 5GB allowance for tethering, they do prorate the charges for tethering your blackberry. Price is anywhere between $15-$30 based on your current blackberry data plan. You can enable/disable the tething feature on your account as often as you'd like.
Charges do get prorated if you start it before your bill cycle and if you end it before a bill cycle. The allowance also gets prorated
Hope this helps