I've moved this thread to the International carrier thread since it's more about pricing with O2 than related to 3G S hardware.
I don't see a lot of O2 users in this forum, so I'm not sure how much feedback you'll get. I see that O2 has
this page up that gives pricing on upgrading to the 3G S. If that doesn't help you, you should try calling them to get information on your specific situation if it's not covered on that page.