Someone hit my car yesterday night or today morning in parking lot.

I thought I would leave it, but found there is paint crack on the horizontal surface just below rear lamp. This will make rust under the paint. If it grows big under the paint, the repair cost might be high. So I want to repair it now.
How much will it cost?

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.