I have a SLK320 2001 which has 81,000 miles and do NOT have an extended warranty. This is how I look at it.
The company offering the warranty at X price must make a profit - so the what they charge you must be higher on average what the normal cost would be to keep the car. It is somewhat of a gamble but a good one. Much like investing in a house or in a good mutual fund - a wise risk.