There are very few alternatives on the M, unless you go custom. There aren't a huge amount of Z4M's out there in comparison to Evo's etc and only a minority want aftermarket exhausts. If you want (not need) a tasty exhaust, you have to pay. It's simple.
I'm rubbish at analogies, so I'll stay away from them and just enjoy the exhaust!
I appreciate your opinion, I would be the same about lots of other stuff like jeans over £100, we are all different.
My justification is that I used some of my xmas bonus for this at circa £600 thanks to the offset of selling my exhaust. To me, spending £600 on something that will bring a smile to my face pretty much every day (I drive lots) and also stay with me for a while, isn't really a huge issue. I can sell it again if I ever need the money and for the happyness it brings, I would have only spent it on a big TV which will be outdated by 3D or some other "next gen" tech. We are on a forum where people spend the same on graphics cards which are worth penuts a few months later, I still didn't expect to come across that opinion within motors!