[Retric]

Average cost to replace a 2,000 sf slate roof is ~$30,000. Someone was likely trying to rip you off.

Lifespan is kind of a leading question as many buildings simply are not going to last. Still, the high end for slate is very high. "There is a chapel in England, built during the 8th century, which has one of the oldest slate roofs known. After 1,200 years of service, the moss-covered roof is still in good condition, demonstrating the permanence of the material."