On some, they've even donated the same amount or more in total dollars compared to OS X users - ie, Linux users have been more profitable even though they have a much smaller market share.
In short, Linux users are very profitable (per-user).
[0] I can't confirm that it's 100%, but it's been every Humble Bundle I've participated in (a lot), which I think is a large enough sample size for the purposes of this statement.