Have you ever planned events in sets instead of a one-time activity? Once you start promoting event series, the scale of the turnout and the subsequent success of each rises to the top. Here’s one way of doing it:
Plan a small event to get numbers that are cheap. Start with the customer lists that you already have. Make sure that the event ends up being a success for that selected small number of people, and that they leave with a good taste in mouth. As a sequel to the event you can up-sell a slightly larger event to a bigger set of people, using the first event’s testimonials and feedback to build hype. Make sure you meet their expectations.