I never am so it's £20/month for about 100GB (and effectively unlimited transfer in the early hours of the morning).
they now also offer a flat 50GB at anytime for £25/month.
you're paying extra for for native IPv6, beyond excellent service, an excellent control panel with options most ISPs don't have, as many IP addresses as you want (optionally PI), custom routing (want an AS for your house?), ability to dual bond across different providers, ability to add 3g as backup link, and much more...
they're truly excellent, here's an example of their customer service on IRC:
1417.59| [bootc] ok, really long shot, but any staff tech around who fancy allocating me some more IPs?
1418.27| [bootc] I already feel rather greedy but I'm using most of my IPs already :-/
1423.30| [@AA-Paul] bootc: What login, and what size block?
1424.14| [bootc] AA-Paul: abc1@a, and a /28 of legacy if possible?
1424.21| [bootc] for @a.1
1425.31| [bootc] I then need to re-number internally and expand my /27 sub-division of my /26 to the full /26 :-/
1426.31| [@AA-Paul] bootc: Done. 1.2.3.4/28 will be routed to you when your router next logs in
1426.57| [bootc] AA-Paul: many thanks, you're a star! have a great rest of your weekend
1427.10| [@AA-Paul] No problem. You too :)
(no I don't work for them, but am an extremely happy customer)