How much should a timing chain replacement cost?

I've had the injectors replaced @ 3 months ago and just had the platinum plugs replaced. I just started noticing that when excellerating, sometimes between 55 - 60 mph I get slight hesitation. I had my mechanic drive it on the highway and he said he could hear a "ping" sometimes which was the chain hitting against the timing chain "housing". I have a 3.8 engine.

I would get another opinion! Can't hurt.

3.8 engine, sometime a slight hesitation and shaking between 55 - 60 mph. Most of the time it's a smooth transition. Just started doing it, have had the injectors replaced @ 3 months ago and just had the platinum plugs replaced.