I've caught the bug as well for quite a while. It's endless upgrading for most of us until we get to the point where our ears can't tell the difference between the high end equipment that you have and the next step up from that. I guess for most people, it is absurd to even spend $100 dollars on headphones, but I think if you look at it from a longterm view and the fact that headphones retain their value relatively well, it's a sound investment if you value hearing good sound.
Before I bought mid-higher end headphones, I used to go through several pairs of those cheap DJ style headphones from Sony, Panasonic and etc. They would last me maybe a few months or so before they broke and I would have to replace them. iBuds, I also have gone through several of those and they get messed up and start crackling after some extensive use. Not to mention the sound from these products can't even come close to comparing to something like a $100 entry level headphone like V-Moda Vibes or AKG K81DJ's.
I think theres a lot more things that are a complete waste of money than a good quality sounding pair of headphones. But, then again everyone values certain things differently. To each, their own.