At its core, b2evolution is a benevolent effort that is not designed to make money. See: About us.
b2evolution is a fairly small project that lives mostly through the dedication of its contributors and supporters. There are no employees, no offices, no commercial structure. Therefore, the project does not have excessive financial needs.
Yet, of course, there are always bills to pay. Starting with hosting this website and outsourcing work like developing features no one else has free time to work on, graphic design, documentation, etc.
So who pays the bills?
For the most part, b2evolution is sponsored by The Evo Factory.
The Evo Factory creates commercial & client web sites using b2evolution. They play an active role in the development of b2evolution.
They feel b2evolution gives them a competitive advantage over their competitors and, putting their money where their mouth is, they make sure b2evolution is sufficiently funded to live on.
This model is very similar to how Firefox is funded for example. Only on a significantly smaller scale ;)
b2evolution also partly relies on donations. Any donations are 100% invested in the development of user features that don't fall into the realm of sponsored development.
This includes features that have been frequently requested on the forums but have found no benevolent developer to work on for several months, sometimes years...