My brother has a Canon 70-300/4-5.6L IS
I have a Canon 100-400 Mark 1 and a 10-22.

He is buying the 10-22 for sure and I just sent him the 100-400 as well to try out. He may (or may not) decide he prefers it to his 70-300, in which case we would swap. (I would rather like a 70-300L, though it seems a bit redundant alongside my 100-400 II. I do use that 70-100 focal length a lot. Or I might just sell it.)

So, what's a fair swap?

70-300 for $100-400 plus $200?
70-300 and $200 for 100-400 and 10-22?
70-300 for 100-400 and 10-22 straight swap?
10-22 for the already-agreed cash price; exchange the 70-300 and 100-400 $100? $200? $300?

Dunno.

Just want to be fair to both parties.