Im using 200m3 gas a month , can please tell me how much I will have to pay

200m3 equates to around 2230 kWh, plus or minus a tad depending on the calorific value for that month.
Prices vary depending on your region, but as a rough guide it would be about £88 in the Midlands (3·94p/kWh).

Indra, are you trying to work out your monthly dd or a specific amount for 1 month (which 198Hz has given you)