Hi, Better late than never. If you have double doors then presumably they are all glass. You will have to get a new glass unit with a cat flap hole. You can by cat flaps for glass. However the unit will have to be toughened and will probabably work out at about �50 - �70 per square metre, plus the charge of fitting the glass and the cat flap. You will be looking at about �150 all together.
If you have a mid rail with glass at the bottom the cheapest way would to get an infill panel with a catflap hole, but this would look a bit daft, unless you put infill panel in the other side of the door as well.
We charge about �60 to supply a panel, with catflap in it
If you are thinking about ordering the glass yourself remember the thickness of the glass unit is important. Regards Al