E3 Tail light $56, E3 Seatpost Mount $132??
 

O.K. I get it, Supernova made a really great tail light that must be worth the crazy price of $56. But they also make the same tail light with an adapter for your seat post that costs twice as much - $132. I get that the light is probably really great and worthy of such a steep price, but can anyone explain to me how an adapter is worth $76 - that's more than the light !?! (See http://www.peterwhitecycles.com/supernova.asp ) What am I missing???

More importantly, I suppose, can anyone recommend an adapter that will allow me to mount the cheaper (regular) light to my seatpost?

Make your own ... There are several ways of doing that. Probably the simplest I can think of is to mount it to a flat piece of metal using spacers as pictured so that the wire can clear. You can secure that vertically to your seatpost by either using clamps, big o-rings, or some old inner tube sections. You may wish to make the metal piece about a 1/2 inch longer on the top and bottom so that you will have enough surface space to secure the clamp.

Make your own ... There are several ways of doing that. Probably the simplest I can think of is to mount it to a flat piece of metal using spacers as pictured so that the wire can clear. You can secure that vertically to your seatpost by either using clamps, big o-rings, or some old inner tube sections. You may wish to make the metal piece about a 1/2 inch longer on the top and bottom so that you will have enough surface space to secure the clamp.

Well I tried to draw it out, but it did not work...so I deleted this part. You'll just have to work it out in your head

I noted that too but wanted the rack mount version fortunately. As a hobbyist machinist I would make my own mount to allow mounting the rack mount version to the seatpost if that was where I wanted the light.

I suspect that the difference in price is primarily due to less demand for the seatpost mounted version. All Supernova lights are expensive but the machining and finiish on the Supernova lights and the taillight are pretty much flawless. Combine that with German labor rates and the current value of the dollar versus the euro and the prices are very high.