Canon's 70-300mm (non-L) is a decent lens. Not quite the reach of the 100-400, but it's under $600. Note that you should avoid the 75-300 lens. That one is junk.
https://www.amazon.com/Canon-70-300m...dp/B01LXTX4WY/
There's also the Sigma 100-400 for about $700.
https://www.the-digital-picture.com/...rary-Lens.aspx