It really depends on the content. If the magazine is offering access to speakers or companies who might be difficult to meet otherwise, or some specialized panels that give you ideas on how to get more traffic or better monetize your blog, then it could be worth it.

Plus, events have some fixed costs like event space, setup costs, etc. that could justify charging for attendance.

If the “conference” is more focused on buying something, rather than learning something, that could be different.

-YM