They want how much for a back extender? I don't think so 'Tim'.
In an effort to do some close-up shooting, I needed a back extender for my 120mm Nikkor macro lens.
As short trip to the local lumber yard scored me a nice piece of African mahogany. Home Depot supplied the brass bits (100lb picture hanger hooks and some brass fittings).
$20-bucks and a few hours in the shop, and voila.. ...a 100mm-back extender.