Charing rent for additional parking spot - West LA Multiplex

[Alfred Moon]

We own a 4 plex in West LA with two 2-car garages and generous street parking.  There is an additional uncovered parking space adjacent to the house which was intended to be reserved for repair persons, contractors, but they always street park.  We have 2 tenants who are asking about sole use of the spot and want to know how much we'd charge.  The lease does not mention this spot at all.  Any suggestions about how to determine who can access this spot and any references where I can get a ball park estimate rent amount? Thanks!