Extended warranties on M3's cost an absolute fortune - can't remember exactly, but when mine was 3years old, BMW quoted me about £1,000 for an extended warranty - that only lasts for one year! I didn't take it out and the car's coming up to 5years old now, so I've now got an imaginary £2,000 saved up if something goes wrong.
As others have said, it depends how much milleage you do and how hard you drive the car. I only do about 5k a year so not overly concerned...
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