Best Property Investors will tell you the following:

"The best way to know the room rate in your area is to ask letting agents around, and clearly specify to them the room quality, ensuite or not, total bedroom in the house, amenities nearby..."

However, you can have an idea about room rates in your area with Property Filter Comparables :)
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Here is the guide:

The Average price per room will be displayed, and you can sort the results by Price, Listing Date, Status Change, etc. You also have the option to view results from the Last 6 months up to 24 months.