If you can find a FTL owner to buy it, other FTL-intended M42 lenses go for about £140-180 or so. (it has a lens locking system that other M42(-auto) lenses lack, so there is a slight advantage to using Olympus' lenses on a FTL vs any other M42-auto lens)
If you just sell it as a M42 lens, I doubt it's worth that much more than the OM system versions - which sell for about £40-60.
You might have best luck seeing if you can find someone on olympus themed forums that is a FTL collector that wants the lens, or try your luck on ebay but make sure that it's clearly marked as a FTL lens (and thus likely to garner the attention of people that know what it's worth). BIN would probably be a good idea, so to avoid people looking for cheap M42 lenses. You might have to list it a few times, although FTL stuff is so rare on ebay that I'd be surprised if someone doesn't snap it up.
edit:
btw more FTL info here :
http://www.biofos.com/cornucop/ftl.html
I'm not sure I'd trust his price guide though, he states £45 for the 28mm f/3.5, which is less than (far more numerous) OM-system 28mm f/3.5s have sold for recently, and £15-25 for a 50mm, whereas there have been some going for £150.
There again, he's a FTL collector, so perhaps he's hoping to fool people into offering cheap lenses
