I’m just taking equipment cost here, this is in Australia (Air conditioning is extremely common here, and basically every unit is a reverse cycle heat pump too,

I’m just taking equipment cost here, this is in Australia (Air conditioning is extremely common here, and basically every unit is a reverse cycle heat pump too, so installation also seems very cheap compared to the US). Just saying if the equipment cost of the whole system is well under $5K, it shouldn’t cost anything like that to replace a circuit board (which as others have said, the part probably costs $100 wholesale to the technician).