Cost of engine rebuilds

I've been pricing engine rebuilds and the costs from 15,000 to 20,000
seem ridiculously high. There doesn't seem to be that many more parts
or labour excesses to explain the prices. Anyone have any insights into
were all the money goes? I am really quite shocked at the high costs
for such a small engine (100-125 HP) going so high when it's old
technology for the most part. Not injected just a carb.