A good ball park figure would be £250 - that's what it cost one reporter who went for a swim in the sea forgetting his rental car keys were in his pocket.
If you do lose them, and do not have a car hire excess insurance policy which covers them being lost or stolen (such as that offered by some - but not all - companies on the MoneyMaxim car hire insurance comparison service) do not get a replacement set from a local garage without talking to your rental firm first. Without their approval you could find yourself paying to replace the keys twice!
Find comprehensive excess cover including key cover on the MoneyMaxim car hire insurance comparison service.
If you have any queries please do not hesitate to contact the MoneyMaxim team.