Very surprised no one has replied so here goes. I am only aware of the 'fairground' 0.25 Buggelspanner with the two convolute springs as power source. I have never chrono'd one but can say that from firing one in the garden you can watch the pellet in flight as it curves downrange. At 30 yards and aiming straight at (as far as I was able) an A4 sized target, the pellet dropped so much it did not reach the target. But then I guess they were only meant to be used at max 6 yards ... or should that be 6 feet ... in fairgrounds.
Price will clearly depend on condition ... some were nicely chromed / nickeled so I will stick my neck out and go for £150 - £400.
Cheers, Phil