Quite a lot, given that I have a VW Passat.

Every time some part falls off (which is about once every 3 months on average over the last 4 years) I figure that's a few hundred less that the remaining parts are worth. The value of the parts that have fallen off on the freeway (underbody heat shield, turbocharger parts, front grill and associated bodywork, lights, etc., etc) would easily pay for an average ARF !