But competition within an apartment building car park will be limited - I'd assume there is only a single supplier today and it wouldn't be economically viable

But competition within an apartment building car park will be limited - I'd assume there is only a single supplier today and it wouldn't be economically viable for a second supplier to build on top so it's a quasi-monopoly. If the building owners are alive to this then they would probably have negotiated a price cap of some form in the contract but I'd be surprised if theres any incentive to cut prices.