That's actually a multi-layered question. Meaning there are a lot of financial variables involved.
How much is your house is worth per square foot (or metre).?
Does making a bedroom from an empty space increase any potential asking price?
Does the home's value, based on the comparables in the area that it's in, allow for an increase in value based on the work done?
When it's all said and done, I would expect the percentage to at least reflect the time, labor and material I put into the conversion.