[elihu]

I would add to that: when presenting their work at a conference, presenters must often pay the full price of admission to attend the conference. This can be in the range of one or two thousand dollars. This prevents anyone not associated with an academic institution with grant money at stake or a well-financed business from presenting their results.

[stjohnswarts]

I think the expense to speak is probably to prevent a lot of quacks working out of their basement from presenting hokey physics talks.