Rent for additional parking spot

[Jessica Furnell]

Hoping to get advice from the experts here...I own a duplex that has a large carport directly behind it. It can easily accommodate four cars. We currently have one tenant using one spot and we rent a spot to a neighbor. We're getting ready to rent out the empty unit, and we include one parking spot in the rent with the option to rent an additional parking space. But what's the best way to determine a fair amount to add to the monthly rate for the spot? Previous tenant only needed one spot so it wasn't an issue. The neighbor rents his spot for about $85/month, but I wouldn't charge the same amount for the tenant, would I? Especially considering that off-street parking in the area is pretty easy to find. Thanks for the input!