There is a huge difference between per square foot pricing between neighborhoods, sometimes $50-$75 per square foot.

I changed a water heater, huge leak in one of the units, electrical issues, and I even had to dig a 3 foot hole under the house and personally fix a pipe.

How accurate is comparing properties by their price per sq foot?

It's $1.50 per square foot, and you can do it yourself.

3 bed, 1 bath, kitchen, living room, laundry room and huge 7000 sq foot lot.http://www.zillow.com/homes/for_sale/53021469_zpid...

That said, you could try finding a smaller/larger home of similar vintage that sold within the past ~year in that neighborhood and figure out the square foot price.

I have found most moderate rehab budgets fall somewhere between $35-60 a square foot.

Your best bet is to go there on an evening and see how the block looks then. in a way you are fortunate because these are the summer months and you can get a good feel of what goes on around that house. if you see a lot of hanging around on porches or a house with a lot of traffic (foot and car), proceed with caution.  

I was curious what your price per square foot is for new construction?

I purchased a 4 bedroom, 1,692 square foot home 4 years ago on a 15 year fixed mortgage for 140,000 in Corpus Christi TX at 5.3 percent interest.