A used Canon, Sigma or Tamron 150-600 are options I think of for your price range. The Tamron was close in performance to the Sigma Contemporary. Tamron came out recently with a G2 version and here are someone's thoughts on it, and links to previous version.
http://www.bestofthetetons.com/2017/02/26/tamron-150-600mm-g2-lens-my-experiences/

Some lens reviews of the Sigma, Tamron and Canon:
http://www.lenstip.com/index.html?test=obiektywu&test_ob=217

http://www.lenstip.com/index.html?test=obiektywu&test_ob=403

http://www.lenstip.com/index.html?test=obiektywu&test_ob=434

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