What will our gas bill be?

We started the meter on 00049 and now it is on 00391 (this is for one month) it sounds quite a bit but we only have a gas top and we have had the heating on twice so please dont scare me saying its going to be over 200 quid or somethikng like somebody did about our electric and it actually wasnt. I have asked n power what it is going to be and they said they do not have the calculator that calculates it out handy! Customer service or what! not