If you have a Casio fx-83es, fx-82es or fx-85es you can upgrade it to a fx-991es yourself by taking the back off, and colouring a chip in with a pencil.
The reason for this is because the chinese education system banned the extra features that you get on the 991 calculator for exams, some time after production started on the 83, and so they allowed the calculators to be easily modified without redesign.
It might not work for your calculator, so check out the link below for how to do it and more information:http://www.thestudentroom.co.uk/showthread.php?p=11860136&highlight=How+to+upgrade+your+fx-82es%2C+fx-83es+fx-85es+to+a+fx991es#post11860136
I have also put a link to a casio fx-83es, which you can upgrade, basically meaning you can get the fx-991 for significantly cheaper.
Not sure if this is in the right place for this sorry.