So Bower manages to cough out 800mm reach at $249.99? That's like $0.3125 per mm. Technology type? Mirror. Mind you, Nikkor used to make mirror lens too! So did Sigma, Tamron and the rest of the gang.
Moving to refractive lens, at 600mm, a mind-boggling ten Gs is required from Nikon. That's like $16.6667 per mm of focal reach! Hey, that's over fifty three times above what mirror lens technology is asking for.