Any new Canon 500/4 would be around $10-12K. ,, Pricks will never decrease.
If the 500/5.6 PF is $3600, I 'm guessing the 600/5.6 PF will be $4200-4600.
Rumor is the 500/5.6 PF is back-ordered because Nikon can't make enough of the PF elements to keep up with production demands. Nikon has not shipped a new "E-version" of their 300/2.8 that they will need for the Olympics in 2020. The 600/5.6 PF and 300/2.8e will probably share the same front element so if Nikon ramps up production for their 300/2.8e, and increases production of PF elements they should be able to fulfill demand for PF telephoto lenses.
Nikon is blowing it big time as there is "huge" demand for their 500/5.6 PF lens. And with Canon probably abandoning the 7D camera line for mirrorless, Nikon could have convinced some Canon or would be Canon amateur/advanced action/wildlife shooters to switch to Nikon just for the 500/5.6 PF lens. A new Canon lens (in this quality range) over 400mm is $9k. Well maybe $7k if you go 400/4 DO + 1.4x TC.
Now if Nikon can actually ship a 600/5.6 PF, in decent quantity at a non-stupid price this summer.........................and finally ship the firmware to give the D5/D500/D850 the ability to use/read/write to CFex (Cfast Express) cards then things will look up the a advanced amateur looking to get into sports/action/wildlife but can't quite afford 500/4 or 600/4 glass. Yep, Nikon blew it. If the 500/5.6 PF came out in quantity early last year.....
I'm still kicking myself for not picking up the D500 ($1700 regular price) when they were on sale last time. $1600 for a D500 + Nikon MB-D17 battery grip. ($367) + Flashpoint TTL Li-ion flash ($179). Current sale is $1500 for D500 body only and expires June 1.