New muffler and exhaust pipe, $400. New tires, $700. Third trip to find out why the “check engine” light is still on, $150. The mechanic is thinking the engine may need a head gasket. Estimate: $1,200. Trade-in value on old clunker? Priceless, in the sense that it has no value at all, not in the sense that it is worth a lot. How do you decide when to trade in a car? Maybe I’ve made all the repairs and it will run for another 98,000 miles. Or maybe this is just the beginning, and it will need the new head …