Oh I agree, production costs will no doubt be far less than they are marketed for, but business is business and if they can sell parts at their sometimes outrageous prices they will! Look at it this way though, people pay far more for a darn plain white t-shirt with a logo on it that you could probably make cheaper yourself. Why? Because it has the logo on it they like and they can't be arsed to do it themselves.
It's an offical Renault badge we are talking about here. Yes, you may well be able to get a non Renault badge for far less, but you then run the risk of it not fitting as well as it should, it may well be slightly bowed and not lie flat. I'd rather pay Renault the £15 knowing that it should fit perfectly and be able to take the darn thing back if it doesn't, rather than putting up with an inferior one that may not do the job as well. It's all down to personal preference though I guess.