You're talking about 7.315m³ of concrete. Some trucks can carry up to 8m³ but you may need smaller trucks (depending on access).
Average price per m³ is around €70. If you get 8m³ it will cost you roughly €560
Ring your local concrete suppliers. They give you more accurate prices. Also, make sure the concrete is the correct strength for whatever is being built.
You are talking of putting in a raft foundation on the cobble stones i.e. you shutter it in at one foot high all round and you'll pour the concrete into it.
Can it be laid on cobbles? Depends on what the ground is like under the cobbles and how much weight will eventually be on the area.
For walls you'd normally have more depth for more concrete. Don't forget steel rebars or mesh (as appropriate) or you'll end up with cracks in your base.
Great FAQ page here by Keohane's Readymix: