Hi Strato and welcome to the eForum
Thanks for your post here.
Yes you can use the phone as a modem with the help of some 3rd party software , which I'm sure our eForum members will be able to help you further with.
You shouldn't receive any further costs other than the cost of your internet pack. However it is worth noting that 1GB on a computer will not last long at all in comparison to how long it will last on a phone. You will be charged £5 for a further 500MB of data should you go over your 1GB allowance.
Please let me know if you have any more questions and we'll be happy to help you further
Thanks
Wayne
eForumTeam