it depends, if its leaking from the evaporator it will be more expensive than a simple hose or a dodgy connection somewhere...first thing to do is to locate the leak, fix it and then nitrogen clean the entire system, change the dehidration unit, vaccum and refill...(and be sure to top up the compressor oil...all this is important because if its leaking it means outside (humid) air got into the system and that tends to contaminate the system (both low and high pressure lines, expansion valve, etc...