While Realtors can't legally estimate or predict cash flow or what kind of return you'll make on your investment, there is a simple formula that can help give you a pretty good idea.
Rental Income + Tax Savings - Mortgage Payments + Operating Expenses = Cash Flow
Operating expenses include property taxes, repairs, insurance, cleaning costs, and fees from rental management companies. What's also important to consider is that property management fees can range from 10% to as much as 50% of your rental income, so it's important to do due diligence when selecting a rental management company.
When buying a condo in Myrtle Beach, SC for investment purposes, remember that calculating cash flow is fluid since equity in the condo will increase each year as the mortgage is paid off, and that will likely mean the return on your investment should increase as well. Calculating the expected cash flow on a condo will certainly help decide if the investment is a good one. But since there are many other factors to consider as well, it's always recommended that you consult with a professional Realtor who knows the market.