Sorry but the only reply I can give is that you will know when you have a Problem as it will not function properly, as to cost, that depends on what vehicle it is that you have which also answers question three
I went to Ford garage on the day when my car juddered. He drove from where I park to the back of the garage and said this is gearbox problem. Having quite novice in this kind of thing, I just assumed he is right. I decided to take my car to another garage recommended by my friend, one will know the problem when the car goes in next wednesday. Until then, I am in limbo!