Many investors use a 10% return rule to price out an investment. You should have a 10% annual return after all expenses. Lets say the business takes in $20,000 , deduct the commercial Insurance premiums, real estate tax, heating and electric, maintenance, and mostly a 1/3 vacancy rate. This fiqure should give you the value of the property times 10. So if the expenses are 10k, then the value should be $ 100,000. As far as 1929 coming, I would rather not have a mortgage, unless you could lease out it like a cheap weekly rooming house.