I think the 20 for the high tier might be light or about right. Using your example, you have 27 legendary dups. Wouldn’t you gladly trade 20 of them for any hero of your choosing? Any hero isn’t exactly what we have, and it might not be worth it the first time this comes around. And maybe not the second. But when Ruby shows up, how will I not go for it?
Comparatively I think the 10 tier is too high. I won’t give up 10 heroes for Reuben, because I might as well wait for someone from the 20 tier. And trading 2 for Guardian Chameleon is probably an easy yes for me (I can always use healers, I don’t play mono in war, and I have plenty of tonics). So the low tier to me should be somewhat lower. Even 5 may still be too high because I’d rather wait for that top hero.
heroes that came to mind quickly