I am very much in the same place as Glenn on this. I speak at about five-ten conferences a year. I am almost ALWAYS paid for my expenses. When the conference charges for admission, and one of the attractions is my presentation, then i also charge a speaker's fee, from $300 (for an event that I can without getting on a plane) to $1000 for a longer trip.

But I would also suggest that if you are invited to speak without any expenses covered, it still might be worth it to you---if you think the exposure to that audience might ultimately help your career.

And if you want to get invited twice, practice enough that your talk is both informative and entertaining. I think I get most of my presentations because I am entertaining as a speaker...and then cover the material!