No, they are not all pretty close, just like the asking price for a house for sale varies (more widely as of late, around here) due to the condition it's in.
Assuming similar condition, area, size, and quality. There is a typical sales and rental price.
The way real estate agents do it is to see how quickly the rental property gets a taker at a given price. If it sits for months with no takers, it's too expensive. Lower the price and see what happens.
That's exactly how owner rent a property too. When it rent out at about the typical time and similar to the recent rental price of similar properties. If government wants to check for the price, and you have to defend the price, you need to give out why you are renting for below market cost to your family member. (they don't care if your number is off to their favor, or if the rental is to an outsider)
If you are IRS you can also check the typical rental revenue that others put in their 1040 schedule E in your area. They know what range it is in. You have to convince them why your rent to your kin is so low if it wasn't in this range because you are being audit when things happen.
You obviously haven't dealt with the subjective whims of those who are looking for a place to rent.
That's their business, and let them pay a higher price for their dream home, but that doesn't means your property is worth less if there are other people willing to rent it at the price you set. I've been dealing with apartment rental since I was 16 and now I'm 31, so I know what I'm talking about and what you are talking about.