It depends on many factors - the industry you focus on, the ability of your attendees to purchase business services, the size of your network etc. A rough rule of thumb is that you can make about $5-$10 per attendee at each event. So for an event with 100 people, BrightMap could pay you $500-$1,000.
Yes, your existing sponsors can participate and no, BrightMap will not charge them. Just have your existing sponsors register on BrightMap and connect to your organization. When they connect to you they will indicate that they are already a sponsor. Once you approve them, they'll get leads from your organization and will not be charged anything by BrightMap.