Mitsubishi Galant 2001 V4, head gasket blown

I bought a mitsubishi mitsubishi galant 2001 v4, before buying it I did a 15 pt checking & found every thing is fine. I also took it to a mechanic to chk the spark plugs and wires as service engine soon was on. He just opend those plugs and cleared it n the light was gone. But after one week again it came up, & again I took it to Autozone, they scaned the code n PO302 causing missfire. They suggested me to chk the spark plugs again. I took it to another AAA certified mecanic, who checked the car & told me that for 2 cylinder head gasket is blown up. So, I need to cahnge it, & that guy give me an estimate of $1200. I am wondering, really how much it will cost for me to replace the gasket because I found gasket head cost max $150. I also want to knoe how it is possible to be sure that the guy is not cheating me. Please suggest me.