Hi,
Can't help you with the failure rate.
But perhaps I can give you a couple of things to think about;
1. If a chip is going to fail it will fail within a relatively short space of time (always dependant on usage, I know). Therefore the point is not about premature mortality rates, because you'll be covered in that period.
2. If a CPU fails it tends (in my experience) to be due to "outside" influence ie. cooler packing up, person attempting overclocking and a host of other situations you can see from posts on this site - the point is: warranties only cover defects in the product (it may be different in whatever country you live, but I would doubt it).
3. Generally speaking it is cheaper to find your own warranty. You will have a years warranty anyway, so what you are talking about is a warranty extended by 2 years for $50 - shop around.
4. How often do you intend to upgrade your CPU. At the moment 3Ghz would appear to be a FAST processor, what about in 12 months?. How close to the latest technology are you intending to stay? Many warranties tend to be like-for-like or repairs.
There is a point 5: about purchasing "goods unfit for their intended use", but I will leave that alone.
Essentially, $50 will buy you peace of mind. IF someone posts that 99.9% of P4's will last 3 years, will you believe them? After all, how long has a 3Ghz P4 been around?
I could say more but I think my point has been made.
All the best. Running a PC without backups is like driving a car without a seatbealt. Your knack'ed when you crash!