We each have a sim-only rolling monthly contract.
Mine is £10 pm & my wife's is £6 pm.
We both only use a fraction of our allowances, especially me as we only get a weak 2g signal so I hardly use any data.
Our dual-fuel bill for 13th Sept to the 12th Oct was £50.89p.
Our highest bill since we switched in Dec 2015 was for the 13th Jan to the 12th Feb 2017 & was £88.95.