How much it costs to replace an airconditioner compressor?

Our central AC unit is having problems starting up in the evenings. In the morning it starts up ok with no problem. A technician came out and diagnosed the problem. According to him the AC compressor is drawing up so much power that it's tripping ( not sure if this is the right word he mentioned ). The only solution he gave was to replace the compressor which would cost $3,100 including everything. I'm not sure if the diagnosis is correct but if that's the case ,does it really cost that much to replace the compressor? The unit is about 12 yrs. old. Feel free to chime in if you think the problem is related to something else and not the compressor. Thanks for any info.