There are several ways to tackle this question, Phil.
First, if you are purchasing the "use" of the spot, say in a building with valet or assigned parking, consider the value of what it would cost you to rent parking nearby. If parking would be $200 per month, then you don't want to pay much more than that amount, even for convenience.
If you are purchasing deeded rights to the spot, you first need to check with the Homeowner's Association to be assured that title can be permanently deeded to your unit. If the person from whom you are considering buying the spot only has one space, you also need to consider whether city regulations allow them to give up their sole parking space.
As to value of an additional space, your Realtor can tell you what the value of an additional deeded space would be, just by looking at comparable sales of units with one space versus units with two, and so on. It depends on availability of street parking, accessibility of guest parking, etc., but in general, units on the Westside of Los Angeles with 2 parking spaces get a slight bump in value at time of sale (perhaps 2-5%), but the real value is in market time. A unit with one parking takes longer to sell, as does tandem parking vs. side by side.
Ultimately, the formula for assigning the value to an individual parking spot is, "What is it worth to me?". Imagine you were the buyer of your unit with that extra parking place. Would it dramatically increase the value in your eyes? By how much?
Finally, be sure that the person "selling" you the parking has the right to do so, by running a full title report. In addition, talk to your attorney and title representative to make sure title is assigned to your unit on a permanent basis.
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