If my aggron uses metal burst to Alakazam its base damage will be its base atk x 1.5= 50x1.5=75 base damage or his normal attack x1.5= 94 x 1.5=141 base damage?
You didn't get it right. Metal Burst returns 1.5 times the damage dealt by the foe's last attack, which means it's basically like Counter/Mirror Coat only it doesn't matter Physical/Special and instead of 2x damage it does 1.5x the damage.
