Discounts could make sense because of the evidence that women tend to be paid less than men, but that would have to be handled with a ton of tact - I don't know how I'd do it.

I've also seen these forms of practical support from conferences that want to encourage participation from women:

* Enacting a harassment policy and/or code of conduct to provide a signal that the organizers care about maintaining a healthy conference environment, which can help women feel more comfortable attending (see http://adainitiative.org/what-we-do/conference-policies/).

* Helping make arrangements for childcare, such as setting up inexpensive group childcare for children of attendees, since women tend to have more responsibility for children than men do.

* Offering advising for qualified people interested in submitting talks who don't have much speaking experience, or having lightning talk opportunities for less-experienced speakers, to help fix the self-reinforcing cycle of few women speakers at conferences.

And pleasantly enough, those policies also benefit attendees of all genders and support other kinds of diversity as well.