Try this:
#Add all GUI Elements to Class
self.amountLabel = QtGui.QLabel('Amount')
self.counterLabel = QtGui.QLabel('Counter')
self.totalLabel = QtGui.QLabel('Total')
self.amountSpin = QtGui.QSpinBox()
self.counterSpin = QtGui.QSpinBox()
[...]
# ACTIONS
self.amountSpin.valueChanged[str].connect(self.onChanged)
self.counterSpin.valueChanged[str].connect(self.onChanged)
[...]
def onChanged(self, val):
#we ignore the val and just get the values directly from our spinboxes
sum = self.amountSpin.Value + self.counterSpin.Value
#and display them
self.totalOutput.setText(QString(sum))
self.totalOutput.adjustSize()