I'm sorry, you are right the problem was with wxSpinCtrl.
This is the code of the constructor:
Cost = new wxSpinCtrl(this, ID_SPINCTRL1, _T("1"), wxPoint(656,136), wxSize(135,27), wxALIGN_RIGHT, 1, 100, 1, _T("ID_SPINCTRL1"));
Here is the documentation:
http://docs.wxwidgets.org/3.1/classwx_spin_ctrl.html#ac5defa94e938dd00380f551502c20a4ewxSmith doesn't allow to set the style wxALIGN_RIGHT that I have inserted in the constructor by hand.
I'm sorry for the error, the first time I have used a ComboBox then I have change my mind and I have chose a SpinCtrl so the problem was with the latter.