The best value for money is the Sigma Tele Macro 400mm f/5.6, with or without HSM. Photozone.de have measured its mtf and found it is much sharper than the Canon 400mm f/5.6 L. I bought one on eBay for £150 in pristine condition and found that is indeed far better than the Canon. Some will autofocus only wide open at f/5.6. But that is not serious as it is very sharp wide open and a non-IS lens is usually used hand held at the highest speeds. My one does focus stopped down but it is hardly sharper at f/8. OK, it is doesn't focus as fast as the Canon. But, it costs less than an extender and gives a fantastic image. THey turn up frequently on eBay, but don't buy the 400mms that are not Tele Macro.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M 18-55, f/2 22, 11-22