I would also check out an Independent shop that can do it for a more reasonable price just in case. The BMW dealership doesn't want to eat the cost of the radiator if they knowingly submit a claim that will be turned down due to outside influences such as a rock. Ask them to show you the hole and play nice. Call BMWNA and plead your case. You never know what they might offer. A dealership can absorb small losses if they wanted to for customer services reasons. I know this first hand more than once from personal experience.

Like others say ask another dealership for opinion or a compromise. But a rock is clearly on you unfortunately and an insurance claim might not net you any better then an Indy shop can install a new one for.