Okay, lets put this in perspective. I have the 500mm F6.3 mirror lens from Samyang, which is sold under several different brand names, and including the T-ring adaptor, I have all of $200 invested in it. That is two hundred dollars, NOT two thousand. For the money invested, I love it.
A 500mm lens from Sigma is about $5,000 and the nikkor is about $8,000, or more. Mind you they are F4 lenses, not F6.3, so you loose the light. Sorry, don't know what the Canon goes for. But think about this - $200 vs $8,000, what does anybody expect for want such a vast price difference? Bottom line for me, for $200 - a lot of fun.
One last thought - I totally agree that the best "mirror lenses" are telescopes. But that is another topic altogether.