1998 Nissan Maxima A/T TCC solenoid valve PO744

This car has been my baby for the past 10 yrs, I just gave it to my son for his 17 b-day, a week later the car is shifting erratically(like its slipping)and there is a loss of acceleration, now the cel came on(PO744), I got a $400 est. Over the phone.I have always done all my own repairs from knock sensors to cats, etc. My manuals say the home mechanic should not attempt this repair(they give no info). What do you think and is the est. I got right. Appreciate your input thanks PAUL