What is a fair commission?

[Cooter Coorara]

Hi all.  I have a full mainland region.  I was diddling around building a downtown, just like a model railroader would do in RL.  Great fun.  One of my builds (I am a builder) is a nice three story building with a store/shop on the first floor and a two story apartment above.  My theme is 1930s. 

I figured that someone may need inexpensive store and living space.  Just like RL.  Perhaps someone who wants to experiment with opening a store.  In RL, I was a businessman in an old mill sown.  Rents were cheap.  Our business folks referred to downtown as an incubator for new businesses.  Well, why not the same in SL?
 

I figured out how much to charge for rent.  Now, here is my question:  I have a couple of rental agents.  What is a fair commission to pay them?  10% of the rental payments?  20%?  This would be an ongoing commission for the life of the rental.  This is a new venture for me and my second go 'round in SL after a long absence, so don't judge me by my age.  I am a noob here.

I pay all of the land's tier and marketing costs, of course.  The agent invests only their time, at their discretion.

Thanks, in advance, for any guidance.