Plusnet's static IP was a one-off £5 activation fee, not a monthly charge.
It was free at one time, I found that I had one by accident and gave it up when they were asking people to do so if they didn't actually need one and started charging a fiver around the same time IIRC.
My IP changes only when the connection drops
(last time it was after 34days). Then I have to edit my BQM