Is there a formula to use to analyse whether an investment property is a good investment?
I know of no generic formula for determining if an investment property is a good investment, besides the trivial formula. Make sure your income is greater than your expenses, and hope the value of the property doesn't drop. Some people will tell you to expect the monthly rent to be a fixed percentage of the purchase price, but that is a goal not a certainty. It is also impossible to estimate the difficulty renting the property, or how long the roof will last. Taxes can't be predicted, as the value of the house increase, so do the property taxes, but you might not be able to increase the rent.  You can't even predict the quality of the tenant. Will they damage the property? Or skip out early? You will need somebody who knows the local market to estimate the local conditions, and help you determine the estimated costs and income based on the actual property involved.