It's going to take tons of hours of work, expenditure, and plenty of risk.

Similarly you can use equity to cover capital expenditures.

Also, don't forget to figure in maintenance, vacancy, capital expenditures (over time it will need a new roof, water heater, ext.), and management into your expenses when figuring out cash flow.

There will be limitations to recovering capital expenditures.  

Using the 50% rule to account for expected vacancy, taxes, incurance, property management, maintenance, repairs, and capital expenditures... the net operating income ends up being $6,000.

Here's a brief example:Gross Rents (Income) - Property Taxes (Expense) - Insurance Premiums (Expense) - Repairs & Maintenance (Expense) - Property Management Fees (Expense) - Interest (Expense) - Depreciation (Expense) = Net Profit, or Taxable IncomeNote that the principal portion of your mortgage payment and capital expenditures are not deducted as expenses.

besides Rental Income per Month, you work out the Cap rate.The very basic calculation that you need to see is the Net Operating Income and the Cap Rate. usually they say cap rate of 10% or above would be a good deal. a rule of thumb the gross rental Income - the overall expenditure( all your taxes, repairs & maintenance & staff salary etc) will give you the NOI.

@Joe KatoThe goal of being a landlord is to get the rent paid on time every month and to minimize maintenance and capital expenditure (capex) fixing the units back up.

Over looking vacancies and capital expenditures that are not present now can truly be a disaster.

The dictionary's definition of invest is:"to put (money) to use, by purchase or expenditure, in something offering potential profitable returns, as interest, income, or appreciation in value."