It varies from provider to provider. A reasonable rate per month would be £15/$30
Either by a minute usage plan or an "All you can eat" 1 time charge per month that allows you unlimited usage.
sky offers unlimited internet usage so no however if you had say 10gb internet usage then you may go over and incur charges
The phone has to have the data add-on so there will be no extra charges due to internet usage.
it means unlimited internet usage.
The DSi connects to your router, the same one your computer will be using, more than likely. Any internet usage on the DSi will therefore be coming from your home internet service. This does not mean an additional charge though - as many devices as you want can connect to and use your router. The only thing to consider is if your service has a monthly usage allowance.
You have to pay for internet usage anyway, that question is irrelavant.
Google has the highest usage.
Assuming you're talking about an internet USB dongle - no. The service provider needs to know who to charge for the internet usage, so the dongle has a SIM card linked to an account.
virgin broadband
It depends on how it is used.
What? iPhone?
Yes there are tons of Internet usage monitor apps out there that help monitor all Internet usage like sites, mails, and chats.