Why not variable parking charges?

I’m a fan of congestion charging for roads, which is the busier a road gets the more expensive it is to use it. The best example of this is in some US cities where there are say two routes to the airport – one which is free, and one with a variable charge so that you are guaranteed to get there in 20 minutes. Sometimes the charge may be $2 and sometimes $15 depending on how high it needs to be to deter motorists.

I wonder if we could do the same for parking. Instead of a flat $4/hr charge, have variable fees. So sometimes it might be $1/hr and other times it might even be $10/hr – the fee adjusting to the level needed to make sure there are always say 5% of parks available.