Myself and my partner live in a complex of flats and houses with allocated parking on the site. We don't have a car at the moment and don't need one. Most people who visit us get the train and there are car parks all around.
Some people who have just moved in have 2 cars and have offered to pay to use our space. Now, for me I would just say, what the heck, use it. However the people who want it and my partner are all very much on the lines of it being a paid agreement.
So, what kind of number do you think is fair?