To get a true estimate you need to get someone to look at it because it depends on the stability of the ground and whether you intend to use it for vehicles or just pedestrians. If the ground is reasonably solid then it should cost roughly £30 - £40 per square metre.
You may also need to check with your local council re their policy on porous materials being used for driveways. In some circumstances a non porous drive would not be allowed.
Thanks - my cheapest quote for this work is £2,800!! It is just an area in front of garage which has previously been gravelled. I think the estimates are way too expensive.