That is a how long is a piece of string question. Without knowing the development, no one could advise on the value of a space.
It is possible to buy them from other owners if and when they come up for sale, but that may be never in some places. If and when they do, there may be a bidding war for them, pushing the price up. Also, while possible to buy, it's nor a simple transaction. There will be legal fees involved for both you and the management company (if they permit the sale) and you would probably be liable for their fees.
In summary, I would advise buying there if you need a parking space. The agent will say anything to get the sale and anything they do say even if lies, they will deny and not be liable for the consequences of their misinformation.