There’s was a comedian podcaster that talked about this and in the coastal areas they had septic systems & the state is requiring sea walls & other regulations before a building permit is even issued!! His number was in line with yours. I was sweating when we sold our last house because I thought I would have to have a $20,000 system installed since we didn’t have the land for extended laterals. The $20k system allows you to dump liquid into a drainage ditch & into streams.
You got lucky good for you