3 cassettes in 50,000 miles would $1200 and 3-5 years of riding.
One of the cogs on a SRAM 1190 is starting to skip under load but it has around 35,000 miles and has suffered to many big-big abuses. I think the key is not letting your chains wear out. When I was lazy and used liquid lubes, my chains would last 2-2,500 miles. I've gotten 10,000 miles on a waxed chain in ideal conditions. 12 speed chains are about $100. So, there is some real money to be saved with the 5 minutes it takes to wax my chain every 500-600 miles. Lets say 20 times per year or around 2 hours. Whats that about $450 per hour?