Make sure it's at some classy venue. People are much more likely to open their wallets if they're someplace like Powell Hall or the Coronado Ballroom that has gold (or at least gilt) on the walls than in a dive bar covered in grime. Even if there is a tip jar.
Emphasize that even though you are charging admission, the money is going to a Worthy Cause that is supported by rich people, like the Saint Louis Symphony Orchestra. Rich people like classical music. It makes them feel cultured. (Not sure where this presumption came from, but it certainly perpetuates itself in movies like A Night at the Opera and Pretty Woman. Yeah, those were opera, but hey, Mozart is still Mozart, right?)
Also emphasize the exclusivity of the occasion by charging a minimum of $750 to attend, though, of course, guests are permitted, nay, encouraged, to pay much, much, much more. That ought to keep the hoi polloi out. Threaten waitlisting, in bold red type on the promotional web site, if people don't get their tickets immediately. Also discontinue, for one night, the most excellent $10 student ticket program.
Arrange some kick-ass celebrity entertainment like Yo-Yo Ma soloing in Dvořák's Cello Concerto and David Robertson conducting the SLSO through Schubert's Eighth Symphony (the "Unfinished" one). Your guests are paying a shitload of money; they deserve the finest you've got, and definitely not any of that super-annoying modernist crap. Only music professors like that stuff, and they're poor.
Corporate sponsorship, like Centene and Enterprise and Ameren UE, helps.
Et voila! Eight hundred grand, from 500 guests. That's net, by the way. Not gross.
'"A Noteworthy Affair' was first of all a fundraiser," says the SLSO's President and Executive Director Fred Bronstein. "It was also another way in which we sought to build the brand and build the visibility of the orchestra. We wanted to create a signature event--a not-to-be-missed evening. And we wanted people to have a really wonderful time. I think we succeeded on all counts."