How much will it cost to have non working windscreen wiper fixed at garage?

You probably need a new motor... if both windshield wipers aren't working. You need to clarify your question.. Do you mean that they don't MOVE or that they just don't work well to clean your windshield?

If you do need a wiper motor, it will probably run you about $150 to $200 U.S. including installation...unless it's a luxury car; then it could be up to $100 more!