We clean it ourselves as well to keep an eye on it and keep costs down. 

Correct, you can't roll repair costs into the loan.

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Add in HOA and utilities and misc costs you are probably at $3500 per month to operate, which is $42000 per year. 

However, land costs are crazy, most lots are going for no less than $400k.

For example, when I get up to “estimating rehab costs”, for me it’s like it’s impossible to know how to estimate rehabbing costs without reading an entire book on it.

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land has very little value if existing homes are selling for less than New build replacement costs. which is pretty much half of the US.. and especially in the mid west and rust belt areas.. building lots have value in dense urban areas where new infill is going on in a big way..

What would you say the cost would be to setup and maintain this type of structure?

One cost to consider is downtime.