Depending on the soil, and if they can hook on to the existing drain field,just a tank replacement can run as little as $2500, but im pretty sure Sheboygan county will not issue a sanitary permit with out a soil eval, $450-600, much of Sheboygan county has a mix of gravel and rock, with some strips of clay, I know of many people that have old systems that last forever, but if they have to replace them, they would not be allowed to install a conventional system to replace it. the best would be to extend the inspection period, and ask the seller to allow an installer to do a soil test and a real estimate. the cost could be $3K or $20K depending on what is needed, the rules are much different than when the old system was installed.

When you start rehabbing higher priced properties, like where I am in Northern NJ/NYC, that 6% could be very significant.I only use this napkin calculation in the very beginning on my first conversation over the phone with the seller to determine if it's worth my time and go see the property (first filter so to speak).

> In "hot" markets, although market cycles are continuously occurring and prices rise a fall... there is an increasing trend line and therefore the next bottom will have market prices that are higher than the last bottom.This is nonsense.I like your napkin drawing, but is it based on any actual data?

Below are a few ways to do a back of the napkin calculation to see if further analysis is worth it.

These are likely done on sanitary manholes and pipe sewers before construction for storm water and sanitary systems of greater sizes (pipes ranging from 15" diameter or above, manholes or sections of 36" or above), not a 4" or at most, 6" drainage pipes coming from a house.  

It's about a 40% cash-on-cash, if I'm calculating correctly (doing back of the napkin math), which is great, but could easily swing drastically lower if I missed something.For me, this amount of cash flow doesn't seem worth the risk.

Using some rough napkin math it looks like both of these are winners.

I ask right then "If someone writes $50 on a napkin and offers do you want to see it"?

What measures do you incorporate in your property management to help maintain that residents are keeping  up their residents in a sanitary manner. 

many have 1 or 2 huge systems for the whole park, sanitary permits are controlled by the county, and you need to have the systems evaluated by a septic person, you can get prior records from the county, also state law requires the systems are inspected every 3 years, and reported to the county.