I'm not saying that others might be, somehow though I'm too cynical to think they do it purely out of the goodness and love in their hearts.
List people who acted on any Stark Trek show who have absolutely no need for more money. I know a few people who are VERY wealthy ... and every one of them does things they probably don't have to do so they can make more.
I think wanting/expecting to get paid for doing it isn't the same thing as doing it strictly for the money, though. The latter suggests they wouldn't do it if they weren't getting paid, and I'm pretty sure of the TPTB do care enough to do it anyhow.
Put together a con, invite the mat their own expense and sit back and count the money -- or be prepared to make the refunds when they don't show. They don't do these things out of the kindness of their hearts or love of ST. Unless someone has some kind of link to Crosby falling on hard times, I think it's insulting to say "well she's obviously down and out or she wouldn't do a con" ... unless you believe that to be true of everyone who does them. Most folks, even "famous" actors, are just like the rest of us -- pay me to go to Los Angeles or wherever for an interesting experience and I'll go. Ask me to foot the bill and no thanks.
Okay, let me put this another way - is it even reasonable to expect them to show up at a con without some form of compensation, even if they're not doing it for the money? It's not like it would be free for them to attend. Your scenario would be more realistic if you suggested paying them strictly for the expenses they would incur for attending.