Boss/drop rate

[LeBarticus]

They say your chances of receiving a rare unit increase as you defeat more bosses. I'd like to receive the specifics on this; I haven't received a rare for probably 10 or so levels, as I just defeated boss 27 for another common >_<

NUMBERS! Where the numbers at?

[Poopenshire]

Toolkit has them. they do increase, but not as much as you are probably hoping.