Comic-Con is the largest con I know of and they could care less if Star Wars characters come or not. Its huge. Bleeds over into downtown San Diego. A real circus. They make everyone pay. If you have a pro pass and work in an industry associated with entertainment or comics, you might be able to get in free.
The 501st gets a free table on a floor that is not near the main sales floor. We are allowed to come in early and stay late because we have to set up and break down the tables and displays every day. But they still charge admission.
One way too, is to volunteer at a convention booth for a few hours. Sometimes the booth owners will get you a pass.
My rule for me is: If you ask me to be a part of an event then I get in free and expect water and a place to change at the very least. I also would hope that my wife and 2 kids could get in too if its an event for children. Like "lego land". Otherwise I dont go. I have been used so many times. Driven hours , paid to get in, fed myself, drove home tired. For what? I dont do that any more.