Refrigerant does costs a fortune, and you can chalk that up to previous versions that destroyed the ozone layer.
The new ones are still bad as greenhouse gases (some are crazy bad!!) but they no longer destroy the ozone layer. It is illegal to vent any refrigerant into the air. It is supposed to be recovered using a recovery machine if possible. If it all leaked out, then there's nothing that can be done about that except finding and repairing the leak(s), pressurize with nitrogen to test for pressure holding, then evacuate the system with a hard vacuum, and recharge to a specified weight.
If you ever have to do the work yourself, the biggest danger is the refrigerant is always under high pressure. Even when it's not running, it has a very high pressure. It will rapidly expand when released into the atmosphere, and will burn (FROSTBITE!!) skin on contact. Dangerous.
Get someone trained to do it, or read like crazy until you know what you're getting into before tackling an A/C repair on your own.