I have done equally well when replacing cheap heavy kit with much better stuff. But where I first got the idea of cost per gram was when looking at saddles. The saddle that came with a bike I had was the WTB Rocket V SLT. Eventually despite loads of gaffer tape it fell apart. I replaced it with the basic Rocket V Race. It was £60 cheaper and weighed 60 grams more. Hence £1 per gram or £1000/kg. If it had worked out at £863/kg it might not have stuck in my head!
Once I had the idea in my head I started to notice it more and more. It doesn't work on everything of course, tyres most noticeably, but whole bikes in a range seem to fall into that category, clockwork ones seem to, not looked at emtbs. I'm not anal enough to have kept records of examples just so I could illustrate my point, but if you look they are out there. I guess that could be just me only looking for items that "prove" my theory - not very scientific I agree. But as a guide it works for me.