how much should it cost to replace an engine coolant sensor in a 2003 mazda 6 as well as a coolant flush?

so my boyfriends car (2003 Mazda 6) is having trouble first his check engine light was on but it was running fine then his car just wouldnt start. the starter was toast so we replaced it but now the transmission is jolting when the car speeds up. We thought it was a computer problem but we took it to mighty auto pro and they said the computer was generating too many codes and the engine also over heated and melted somthing so they would have to preform an engine flush plus replace the engine coolant sensor. They said the total including labor and parts would come out to be about $530.00. I feel like thats a little outrageous but I really have no clue so thats why I'm asking.