Well it used to be 9p per mile but its now just over 10p a mile.
To be fair there are other costs.. LPG servicing is £65/year
And i still have to put petrol into the car - petrol is used in the warm up stages as you drive off and it then seamlessly switches to lpg. Thus more petrol is used in the winter than spring/summer/autumn.
You need to keep at least 1/4 tank of petrol in the car to allow the petrol fuel pump to remain lubricated (this remains running even when your on lpg), In the winter the car uses £30 of petrol every couple of months, but in the summer it barely uses any petrol so i have to put a squirt of stabil in the petrol tank to help stop the petrol going stale.
Now if you were really taking it too far you can over-ride the car starting on petrol and make it start on LPG but this isn't recommended as i think it may risk some longer term damage to the ignition system.