They want how much for a lens extender? Actually, they're pretty cheap. But so am I. A trip to my favourite plumber scored me a few ABS pieces, some glue, and a small piece of aluminum plate.
I fabricated two screw-on lens boards sized to accommodate both copal-0 and copal-1 sized lenses.
Works just fine, but it's a bit of a pain to mount the lens. I much prefer my back extender.
Total cost, $12-bucks.