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From their site: Using our easy-to-integrate API, you can request information such as low, median, and high rental rates and comparable market data for a specific property or area.
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There are no source code libraries or how-to links for Rentometer. If you know one why not add it?
I created a average rental tool if anyone is interested giving it a try.
I think it's a very useful mashup that helps you not to pay for rental more than it costs.