The key point is that it is damage to your property. Regardless of the damage done it's *your* car and they threw a brick at it.
There's the plane of glass and the pain in the ass of fitting it, assuming you didn't get it done professionally.
Last time I spoke to a certain company (which will remain nameless) they were under the impression that my time was worthless (under the assumption I was stupid) At my last place I was charged out at £3000 a day for consultancy. If it took me a morning to sort out said damage and I were doing it professionally I'd be invoicing them for a grand and a half. Compared to that, £50 seems reasonable for what was a deliberate attack against your property.