Dang ! Is it a special septic system you're having installed ? A new regular septic system here 1000 gallon tank with field likes is about $3000.00 and that is with perk test and permits
Used to be like $3K here also but now it ranges in the $10 - 20K.
Shucks even in the $3K days we could make our own and I did so and the ones I made still operate just fine and pass all tests.
Now we need soil analysis, perk tests surveys and at times need to truck in 'certified sand' from afar.
All those tests run $500 each and surveying the neighbors well locations add up as well.
Today you need a 'soil engineer' compared to the old days where you dug a hole and poured 5 gals of water into it and timed the absorption with your watch.
The old fashioned field is no longer the standard now we have 'bionest' 'ekoflow' and next will be, I presume, a computer controlled system.
\Already some fields have 'inspection ports' and another system has some re circulation pumps to assure even distribution of the effluent.
Add to the equation that contractor needs to be 'certified' as well.
What used to be a $25 permit is now, like, $500 or so.
Its all about saving the environment you understand.