What do you mean by living comfortably? I'm assuming the essentials already paid for includes decent housing, a car, proper food, utilities etc.
So how much for extras? Clothes, nights out, holidays, eating out, books, swimming lessons, haircuts, the gym, alcohol.....
Having been in the position of having nothing to spare at all I'm grateful I can afford my gym pass, to grab food on the go, eat out occasionally and even a holiday this year. I would say that was around £200 per month for just me, taking kids and partner into account would change that figure.
I've got simple tastes though, don't care about clothes, am happy to get a basic cut at village hairdresser for £15 every few weeks, don't drink, have a PAYG phone I put £10 on most months. Others would easily spend that on a couple of items of clothing.