I'm not quite sure what cabbagetowner means by "rent ratios".
When speaking of income-producing residential real estate (not basement apartments, but things like triplexes and larger), there are two measurements that can be used.
The Gross Income Multiplier (GIM) is the relationship between sale price and gross income, expressed as a multiple of the gross income. Example: Given Gross income $60,000 / yr., and a sale price of $600,000, the GIM would be 10. This multiplier typically ranges between about 8 and 15.
The Capitalization Rate is the relationship between net (not gross) income and the sale price. Net income (often called the Net Operating Income or NOI.) is the gross income minus the operating expenses (taxes, utilities, insurance, etc.) It does not take into account the financing costs (mortgage). The Capitalization Rate (often called the "cap rate") is the ratio NOI / Sale Price. It's usually expressed as a percentage.
Example: Sale price $600,000, Net Operating Income (after operating expenses but before financing costs) $40,000, results in a cap rate of:
$40,000 / $600,000 = 6.7%. The Cap Rate for good residential properties is typically between about 5% and 9% in the current market. This rate reflects risk, and those properties which are considered to be "higher risk" will reflect a higher rate.
The capitalization rate, being a ratio, can be flipped around. Example: You are considering the purchase of the above-mentioned property, which has a Net Operating Income of $40,000. It's listed for sale at $600,000. You want a return of 8%, not 6.7%. You would be willing to pay $40,000 / .08 = $500,000 maximum.
It gets more complicated than that, but that's the basic idea.
Before considering such a purchase, do your homework!
The Gross Income Multiplier is a quick and dirty indicator. It does not take the expenses into account. The Capitalization Rate takes expenses into account, and is therefore a more accurate indicator. It's also harder to calculate, as more information is needed, preferably extending over a multi-year period to smooth out unusual fluctuations in expenses.