problem with clio immobiliser?

i have a renault clio expressions 1.2 on a 51 plate been having problems with it for while now its been in and out of garages like yo yo and keep getting same ans battery just had it to garage again for diagnostic and been told that problem is with immobiliser for some reason immobiliser is draining power out of battery has anyone got any idea why this happening or any idea how much it costs to get new immobiliser fitted if taken to proper renault dealers