It can be both, though. People who own land and build parking lots and structures on it will charge however much they can, based on what people will actually pay. If parking is more scarce, and the demand is there (as in, it's a popular place to visit), they'll be able to charge more.

And sometimes city councils restrict the amount of parking that can be built because they want to create that scarcity to encourage people to take transit instead of driving, because they know that the owners of the (fewer) parking areas will charge more, making it less desirable to drive and park.