The chances of a commercial flight crash are about 1 in 1 200 000 - pretty low. If each plane makes about 40,000 flights in it's lifetime, about 1 in 30 airplanes will be involved in a crash before it reaches retirement.
Is this an accurate estimate? 1 in 30 seems like quite a large amount of airplanes to crash before they are retired.
I'm also assuming here that after a crash an airplane is written off. This may not be the case - I don't know how much a plane can be repaired.