We can't answer that question because the car isn't where we can see it, so start with some facts...
The belt broke when you were trying to start the engine, so it wasn't running when it broke.
The pistons will have impacted the valves, so the valves will be damaged, how badly and how many we don't know.
To properly assess the damage the mechanic will need to take the head off. This is when the labour charge will start.
The mechanic may find that the damage is minimal and say that it's cheapest to replace the valves and put the head back on. Or he might find that the damage is catastrophic and say that you need another engine... either way you're paying him to work on the car.
This is where my original suggestion of getting an engine from the wreckers came from. I was suggesting you skip the diagnosis and go to the replacement, which may or may not be required anyway.
You with me?