How much to charge for a parkin on a hill where it is difficult to find parking.(Northern California)?

I have an empty car port that I own and I live on a hill in northern california (marin to be exact) where it is often difficult to find parking. One of my Neighbors offered to pay me so that they could use the parkin. I don't want to charge too much because They are my Neighbors but I also don't want to charge too little. (No less than 75$ a month)