I ask my clients for a 50% nonrefundable deposit on anything exceeding $250.00, however few of my jobs go over a couple of grand. It mostly covers materials purchase and pick up. It's really up to your contractor because we/they aren't going to get shafted on signed contracts with an order to start then tie up our own money in materials for a job the client may back out of. Like home builders everything is written in their favor. They will determine what they want.