Where are you planning to get it from? I ask as the quality varies. Some security glass istead of shattering just folds into two, making it even easier for the thief when they finally break it. The thing to remember though is that if someone wants it they'll usually have it, if it involves prising your door open then so be it. I personally would save the £500 knowing that a smashed window will only cost about £100 to replace.