You could look on homegate to see how much parking spaces in your area rent out for - I'm sure this is where the landlord will advertise the space

I can understand where the landlord is coming from as there is now a requirement to have parking spaces for relating to the floor space of the appartment - I'm not sure if this is a recommened practice or law - but they have to build these so of course want some money back for them as they would no doubt have preferred to have an appartment instead of a large parking area.

If he still wants you to rent it ask if you can sublet it and put it on homegate - though it is useful to have a parking space if you have guests - or even for things like bikes, buggies etc (if its in a garage)